Reset Tooltips ? easy (?) newbie question

[kakuta Safe-mail net]

Hi everyone,
              I'd be grateful if you could help with a quick question regarding 'tooltips', is it possible to 
re-set the tooltips somehow?
 
I have successfully created and set some tooltips, but I then want to change these tooltips while 
 the program is running (depending on a certain gtk button being 
 pressed). I am a relative newbie with C programming, and an absolute beginner with GTK programming so please 
spell out any recommendations 
step by step if possible !
         Many thanks,
                    Dan.