Thenk you both. Your suggestions helped me to solve the issue.
First, reading the link by Nico (and before him by Ron Tapia privately) 
recalled me what passwd command does.
Then jdow's post prompted use of sudo.
Actually I had already tried it and it worked.

It was not obvious, haowever, since sudo's allowed actions depend on the 
security policy and I didn't remember anything about that.
Once read 'man sudo' I checked sudoers file and found I had configured
%elio    ALL=(ALL)    NOPASSWD: ALL
(elio is my account's name).

All that I had forgotten, of course :)
Bye
-- 
Elio Fabri