I just got the catalina update for my imac ,and my iPhoto doesn't work anymore , it said that i have to update but i don't know how ? , could you help me ?

iPhoto was discontinued. You need to move to Photos. (the program with the icon like this:

)

Launch Photos with the option key held down. From the dialog, choose your iPhoto Library. Photos will create a new Photos Library with your iPhoto media. Your iPhoto Library will not be changed, though it will be renamed.