Is that a clay mridanga or the USA fibreglass version, clay mridanga;s are hard to fix usually best to discard and get a new version, the fibreglass versions usually have a fix up path that requires glue and fibreglass etc....

mishra:

I don’t know how to change the big patch, someone told me i have to wet the cords, to change the patch. can you help me please?
Also how to tight it as it sounds like a tin can now :(

Yes, wet the leather cords, that will loosen them up, then untie the cords and put on the new head while the cords are still moist.
If the drum sounds like a tin can it means that it is too dry (too tight) - need to moisten the head to make it sound lower. You can use water or a special moisturizer for drums (some hand lotions work well too). ---------- Good luck, as it takes a bit of practice and patience! :)