Let me get this straight:

You have a string of notes, of different durations (e. g. Half - Eighth - Eighth - Quarter).

And you need to change their durations so that they all have the same rhythmical value, e. g. Quarter value (= Quarter - Quarter - Quarter - Quarter).

- Right?

I would use the Speedy Entry Tool.

Position the cross-hair on the first note value (= the Half).

Then, hit the number 5 key.

The Half will change to a Quarter, and the crosshair will move to the next entry.

Keep going until you reach the Quarter.

Do not hit the number 5 key when you are on the Quarter.

Instead, hit the RightArrow key, to move to the next entry.

thanks but this work only bar by bar... if the notes are many...?

In that case I would use the utility.

Utilities menu > Change > Note Durations…

Unfortunately the utility can not do it all “at one fell swoop”.

You have to change one rhythmical value at a time:

First all the Whole notes.

Then all the Half notes.

Then all the …

And watch multi-voice measures and empty measures to make sure things stay aligned.

no jw plugin?

Not knowing the context, the number or staves concerned, the way the notes are distributed in the measures, the only additional idea would be in case several staves should share the same rhythm. In this case, fix one staff first, then use JW Rhythm Copy to apply the same rhythm to other staves.

the context is as basic as possible: imagine a melody made by different note values that I want to make all of the same note value (duration)...

… no jw plugin? …

Try the plug-in JW Change:

JW Change > Note Entries > Duration