I know what transaction malleability is, but I don't understand how someone can change my signature so that it would remain valid. For example, If I want to change signature what part should I change, can someone explain this to me? or is there some example of this, I tried to find it but failed.
A signature consists of two numbers r and s. You can change s with N - s (N is the elliptic curve order) and it would still be valid.
Sources of malleability are documented here https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki#motivation