How to repair a malfunctioning shutter release button?

When I push down the shutter release after achieving the autofocus, the camera again begins the focusing cycle, and then the shutter release happens.

Also, 50% of the time, the autofocus seems to be active, with the lens constantly moving (this is expected only in MACRO mode).

I am observing this problem only since last 3 months (I have been using this camera since 2011). I believe the shutter release button has developed some fault. How can I rectify it ?