How to replace the power button

Lawang Kayu, there are two parts to the buttons; there is the little circuit board that the switch domes are mounted on, then there are the physical buttons you push to activate those domes. It's not clear from your description which part is the problem so I can't tell you exactly how to fix it.

That being said, even though iFixit doesn't have a guide to repairing your phone, there's a Spanish site called Nadie Me Llama Gallina that has guides for both repairs - exactly what you need. It is, as you may guess, written in Spanish but has language translation available so their guides are quite usable and well done. Here are links to guides for both the electrical and the mechanical parts.

Manuales / Xiaomi Redmi 5 Plus / Side push buttons - Nadie Me Llama Gallina

Manuales / Xiaomi Redmi 5 Plus / Side keys - Nadie Me Llama Gallina

As I mentioned, I can't tell you which one is going to fix your problem, but you're covered either way.

Good luck and let us know how it turns out!