File Type Groups Change button greyed out

Hi There

Regarding adding file types to file type groups.
The "Change" button is greyed out.
Any ideas?

File Type Groups are specific to Opus. The "Open With" concept only applies to system file types.

Instead, double-click the group to edit it, then change the Left Double-Click event.

Thankyou Leo.
I was totally confused.
To be clear what I was wanting to do is add an additional file type extension to the Text File type group.
This has nothing to do with the Change.. button.