How do I repair my Computer Scroll Mouse ?

I have bought a "PHILEX" (75240HS) Scroll mouse from a ASDA superstore in UK. It worked very well until I experienced dodgy things begining to happen. Now it is unable to navigate throughout the desktop as I wish and sometimes it stops. Then I have to lift it off and hit it hard in the mouse mat in order to begin it working.

Please instruct me how do I fix it to keep in working order. And the reasons for these things happening. Extra precautions to keep my scroll mouse in optimal condition.

Your hints and advices are gratefully appreciated.