It isn’t always advisable to edit dimensions, but if you want to show a value that’s different from the actual distance represented by the dimension, you can double click it, then overtype what is there. But be aware that will disconnect the value from any change in the referenced geometry to which it refers.
If you want to add text to the dimensioned length, but keep the actual length value connected to the geometric distance and any subsequent changes in it, replace it by
<> (that’s open angle bracket, close angle bracket) plus whatever text you want to add.
If it won’t open for editing by a double click, you have a problem with your installation of SU.
Try re-running the installer (exe file on Windows), but R-click on the file, and choose Run as Administrator, then Repair, and try again.
Best wishes for your learning curve.
Did you mean instead, the dialogue to customise dimensions in Window/Model info? This one, in SU 2017?
It will look slightly different on Windows, but contains much the same set of choice, I think.