Apple Watch Battery replacement

not sure if that is really a user replacement thing

will surely void the warranty but with a s2 model that old warranty would no longer be a thing

try reading Apple Watch Battery Replacement - iFixit Repair Guide

more apple watch replace battery - Google-

that being said it's an 10 years old model and will not support the newer watchOS versions and nor likely many apps

Better make sure the extra battery is good. Sitting there for 6 years it could loose its capacity.

I have an Watch series 6 which has battery a bit less than 80% which Apple says is the time to replace it. When I asked about a replacement they said it was $99. However, for some models they said they may replace the entire watch with a refurbished one.

Just some food for thought.

not sure what you bought but that type of battery is not any type of battery that is used with any apple watch