An uninstall and reinstall would not change anything. UltraEdit does not register itself as a default application for a specific set of file extensions as it can be used as general text editor for hundreds or even thousands of file extensions.

The solution for associating file extensions with UltraEdit on Windows 11 is as follows:

1. Open Settings - Apps - Default apps.
2. Enter at top under Set a default for a file type or link type a file extension like .cpp.
3. Click below on Choose a default.
   There should be listed UltraEdit Professional Text/Hex Editor and UEStudio if UEStudio is installed too.
4. Double click on the UltraEdit or UEStudio list item or click on it just once and click on button Set default.

The steps 2 to 4 can be done multiple times for other file extensions.

If there is missing UltraEdit Professional Text/Hex Editor respectively UEStudio in the applications list although being installed, there can be scrolled down in the applications list to the bottom and clicked on the last item Choose an app on your PC and browsed in the opened Open with… dialog window to the executable %ProgramFiles%\IDM Computer Solutions\UltraEdit\uedit64.exe respectively %ProgramFiles%\IDM Computer Solutions\UEStudio\uestudio.exe or wherever UltraEdit/UEStudio is/are installed. Double click next on the executable file. Then UltraEdit or UEStudio is in the application list and step 4 can be performed for associating UltraEdit/UEStudio with the entered file extension.

---

Some technical background information.

UltraEdit adds on first start after an installation to the registry hive of current user:
The installation path can be different on having installed 64-bit UltraEdit into a different directory than the default installation directory.

These lines could be also copied and pasted into a text file with adding as first line REGEDIT4 and saved with a file name like UltraEditAppReg.txt or UltraEditAppReg.reg (file extension does not really matter) into a directory like %UserProfile%. A command prompt window can be opened next and executed the command line reg import "%UserProfile%\UltraEditAppReg.txt" for importing the application registration for the current user. Local administrator privileges are not needed for the application registration. The created text file/registry import file should be deleted after the successful import with del "%UserProfile%\UltraEditAppReg.txt" as no longer needed.

But such a manual registration should not be needed at all for getting UltraEdit Professional Text/Hex Editor listed in the applications list as it is done by UltraEdit itself on first start after an installation.

---

UEStudio adds on first start after an installation to the registry hive of current user:
The installation path can be different on having installed 64-bit UEStudio into a different directory than the default installation directory.

These lines could be also copied and pasted into a text file with adding as first line REGEDIT4 and saved with a file name like UEStudioAppReg.txt or UEStudioAppReg.reg (file extension does not really matter) into a directory like %UserProfile%. A command prompt window can be opened next and executed the command line reg import "%UserProfile%\UEStudioAppReg.txt" for importing the application registration for the current user. Local administrator privileges are not needed for the application registration. The created text file/registry import file should be deleted after the successful import with del "%UserProfile%\UEStudioAppReg.txt" as no longer needed.

But such a manual registration should not be needed at all for getting UEStudio listed in the applications list as it is done by UEStudio itself on first start after an installation.

---

The installers of some applications register in the registry hive of local machine on application installed for all users or in the registry hive of current user on application installed just for the current user with which file extensions the installed application can be associated by default.

Here is an example of such a registration on installing 64-bit WinRAR for all users: HKEY_LOCAL_MACHINE is HKEY_CURRENT_USER on installation of WinRAR is done just for the current user.

A compression/decompression application like WinRAR can be registered with a dedicated list of supported file types/extensions. WinRAR archiver is therefore listed already in the window Apps > Default apps and the user can click on this list item and choose to associate WinRAR with all these file extensions. But that is impossible for a general text editor like UltraEdit for which nearly every user has a different list of file extensions which should be associated with UltraEdit for opening or printing a file with a double click with UltraEdit.

There can be executed the following command lines in a 64-bit command prompt window on 64-bit Windows to see all application registrations: