I think my Mi 4i battery is faulty (battery draining fast) and slightly bulgy. So, i want to DIY exchange the battery. Please clears my confusion or advise me, thank in advance.

1. Should i got to MiStore first? I dont know how they works and i just rooted my phone.
2. Should i go to shop instead for replacement? But i afraid they charge alot (>RM100) and i am not sure it's the battery fault.
3. Where should i buy the good replacement battery? I scoured Lazada and found out there are 2 price ranges (~RM50 and ~RM90). The seemingly original (labelled with Xiaomi battery and such) is the cheaper one while the other screams Made In China but the user comment is nice. The manufacturers are different but the battery model (BM33) is the same.
4. Is DIY easy? Is it easy to damage phone during DIY replacement? I am newbie with clumsy hand. I watched the tutorial and all i know was screwing and unscrewing thus cannot gauge how hard and delicate the operation is. Anyone has experience?

Thank you for reading.