I suggest reading first How to search the current selection in internet via UltraEdit command? See also: Does UE support configurable external help (e.g. for programming languages)?

The key F1 should open the help file of UltraEdit. That should work also on currently latest Windows 10.

What does happen on double clicking on the file uedit64.chm in the program files folder of 64-bit UltraEdit which is by default the folder %ProgramFiles%\IDM Computer Solutions\UltraEdit?

That should open the help file like pressing F1 in UltraEdit or clicking in menu Help (German: Hilfe) on first item Index (German: Hilfethemen) with traditional menus or Help (German: Hilfe) with contemporary menus. In ribbon mode the Help is opened by clicking on symbol on right side of ribbon bar showing a white question mark in a blue symbol.

The key F1 cannot be really reassigned to a different command. Windows applications based on Microsoft's GDI or GDI+ libraries have built-in the feature to open on pressing F1 the help page associated with a dialog window if a dialog window is currently opened and it has a help identifier associated which is assigned in the CHM file to the appropriate help page. Open the Replace window with Ctrl+R and press F1 to see what I mean.

Shift+F1 is a key assignment which enables by default the help pointer. This assignment can be changed as the enable help pointer command is not really useful anymore in UltraEdit for Windows v27.10. This command can be still used in toolbar/menu mode to enable the help pointer and click on an item on a toolbar to open the appropriate help page for the command, but I suppose that nobody uses that help feature nowadays. So it is possible to remove the assignment of Shift+F1 from command Enable help pointer (German: Kontexthilfe aktivieren) and assign it to a different command like User help 1 (German: Anwenderhilfe 1).

Well, the default key assignment in the used GDI libraries of Microsoft to open appropriate help page on pressing key F1 working also in most of the configuration windows can be confusing on pressing key F1 with document window of file having currently the input focus. There is the default help page of UltraEdit help opened on nothing selected in active document window on pressing key F1. But if a string is selected in active document window and there is one user added *.chm file, this help file is opened with searching in help index for the selected string. If there is selected a string in active document window and there are two or more *.chm files or URLs added by the user, UltraEdit opens a dialog window to select which user added help to use. The reason is that although F1 has a default action associated, UltraEdit overrides this default action for active document window by assigning key F1 to the command with internal identifier ID_HELPSEL which is not listed in the commands list in key mapping configuration dialog nor in the file created on using the button Show key mapping in editor in Key mapping configuration window.

I detected an issue while writing this post which I have not seen before as my configuration has several *.chm files added. If there is a string selected in active document window and there is just one user help added which is a https URL and not a *.chm file or a http URL, pressing key F1 opens the window Windows Help and Support which is the online Windows/Office help. The same issue exists with multiple user helps configured and the selected help is a https URL and not a http URL or a CHM file. So UltraEdit does not correct handle this use case and the underlying default help behavior of GDI+ library also used by Microsoft Office is the result. I reported this issue to IDM support by email. The solution is using the http URL and let the web server redirect to https or click in menu Help on the menu item of the help with https URL or use the User help commands.