The context menu on Sessions tab of File View contains some commands for organizing sessions, but it is not possible in UC v17.00.0.18 to change the order of the saved sessions. Therefore I suggest to send an enhancement request email to IDM support.
In the meantime you can open %APPDATA%\IDMComp\UltraCompare\UCSessions.dat in UE/UES while UC Prof. is not running which is a UTF-16 LE encoded file with UNIX line terminators (just line-feed) with an ugly formatting because of using two line-feeds per line.
The file contains <GROUP_BEGIN> and <GROUP_END> to mark begin and end of a group and <SESSION_BEGIN> and <SESSION_END> to mark begin and end of a session within a group.
To change the order of the sessions within a group, move the session blocks in the file within the group. Save the file and start UltraCompare to see the sessions in order as listed in the file UCSessions.dat.