Here is a simple tutorial with which you can reverse your mouse buttons. This can be done by simply going to Control Panel also but I am writing this tutorial only for knowledge sake.
Lets Start :
First click on Start Menu and select RUN. Now type regedit and hit Enter. Registry Editor will open
In the left hand side panel click on HKEY_CURRENT_USER >> Control Panel >> Mouse
Now in the right hand side panel look for SwapMouseButtons. Double click on it, in the Value Data, change the entry from 0 to 1 and click OK.
Now reboot your PC and you are done. :)