How to fix my overheating iPhone?

My iPhone keeps overheating and running out of battery super fast. I took it to the Apple store and they were able to identify the issue. They said it was due to my phone acting as though everything was in use even like it was "making a constant phone call" even though I closed all the programs and stuff. They were not able to do anything about it unless I paid a ton of money. They also mentioned there was slight water damage probably from it sitting in the bathroom while I showered.

I don't have the financial means to replace my phone or even get a new different phone for that matter. So if someone can tell me how to resolve this issue or at least alleviate the overheating at least slightly or how to improve the battery life cause the 5hrs or less isn't cutting it at all for now
Any advice you may have I would appreciate being able to do it without forking out tons of cash

Cover it in dry white Rice for 48 hours 2 day to suck out any moisture. See if that helps!

First turn off your cell phone and now put this into a place which is air tight and then wait for 3 to 5 hours after this check your cell phone hope it helps
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