Changing doubt motherboard
 

I have two iPhone 4, one blocked with iCloud, everything works.

The other with broken screen, internal oxide by water damage and battery that lasts very little, (not using it also consumes battery power), the rest everything works.

I would like to exchange motherboards.

It can be done smoothly, What precautions should I take.

I accept advice and suggestions.

My English is not very good

https://www.ifixit.com/Teardown/iPhone+4S+Teardown/6610

just be careful when you disconnect cables and dont lose 3 little metal parts (one on battery cover, another near headphone jack and another near power button).

ouch, wrong link, thats ok:
https://www.ifixit.com/Guide/iPhone+...placement/7283

Take it to a repair shop.