You can select a single measure stack by double-clicking in any of the measures. THEN, go to the time signature dialogue by right-clicking in the stack (you have to click in a measure, not in the space between staves.)

Select one or several measures, then right-click > Edit Time Signature, which will give you a screen where you have all the choices you may want (one measure, several, or until next time change) regarding region as well as type of signature

yes and no...

I can edit that bar Time signature if the chosen TS is in the contextual menu list, otherwise if I have to choose Edit TS... is

Measure X to the next time change

and not, more logical

Measure X through Measure X

in other words: 1 measure does not work as a region

The question may have been misunderstood. Alessandro wanted in one click to have the single selected measure also to be the selected measure in the dialog box. I looked into the help files (Keyboard shortcuts) and didn't find a way to achieve this in one step.

is right Michel!