If they're rivets, the only way to take them out is to cut them, potentially damaging the case.
I am sure they are rivets. I am confident about it. Now, how to open the other two sides ? bcoz i cannot take my motherboard out nd it makes attaching nd detaching process difficult. Should i buy a new cabinet 
How often are you inserting and removing things inside the case? Unless you were putting something under the motherboard (which you shouldn't be), there's no reason to take it out unless you were replacing the motherboard.
Some cases do not open on both sides. Some have a bracket the motherboard is attached to that comes out from the one open side.
Still not clear on what you're trying to do. Could you supply some pictures?
