You can do this at home also if you have the required tools or else you can visit a local mobile repairing centre and get is fixed for few hundred rupees. If you take it to the Samsung Service Centre they'll charge you more.
Just watch the video & follow the given instructions:
My M21 was lying dead for about six months because of the restarting problem. But I watched this video & tried to fix at home it resolved the problem. After the fix I updated the software and it restarted for few times & I had done factory reset. Now my M21 is working absolutely fine.
