How to fix a rim, that bent vertically inwards? The rim is mostly round, but in one place it's bent a little bit (3-5 mm) towards the center. 

I only found manuals about fixing wheels that are wobbling side to side, but nothing about vertical bendings. 

So my questions are - is it worth to fix it or it's better to just buy a new rim? If it's possible to fix it - how to do that - just by adjusting spokes or by removing them along with the tire and straightening the rim itself?
Vertical adjustment of a wheel is exactly the same as adjusting for wobble, except you loosen the nipples going to both sides of the hub rather than tightening one side and relaxing the other side.

Wheel truing is an art that can be explained but not learned without doing.

Most wheel truing stands have a way to measure "out of round" as well as left/right wiggle.

Depending on what happened to the rim, it may be stretched or dented in that spot, or it may require ridiculously spoke tensions to pull it back to "round" 

You have to make the judgement call about how bad it is and whether replacement is required.