How long ago did you bought the mouse? Have you tried asking the shop where you've bought it from? For some smaller peripherals (such as keyboard, mouse, etc), depending on the manufacturer/brand/model the warranty period could be much shorter (often less than a year).

Have your tried disassembling the mouse, and spray contact cleaner into the button?

u r in luck buddy. i used to handle logitech RMA. the procedure is quite simple. first u need to contact support via website at logitech.com. fill the form, then they'll respond to you, telling where can u send the mouse. then u just print the reply they give, bring it to any address they state there. mind u, there will be no on-spot replacement as your item is over 1 year. u might have to wait atleast a week. but the good thing is, u'll get a brand new mouse. logitech is very caring about their customer. they'll never repair it.