I just did this on a Polar F6 unit- the four screws are not the problem, unless you drop one.
The battery is held under a metal clip contraption that's quite fidgety. Just go at it easy, gently prying each corner of the clip up and try not to bend anything in the process.
Keep all the pieces organized as you take it apart, and be sure you can put it all back together in the right order without bending anything or introducing any fingerprints or dirt to the inside of the unit.
It's entirely do-able if you have any mechanical skill and some patience. Otherwise, it might be worth a few bucks to have the local watch people take care of it for you. There's no need to send it back to Polar, unlike the strap when it needs replacement.