how to replace leaking tub/shower faucet stem

As shown in your photograph, “MIXET” is the brand of faucet.

Apparently “Brasscraft” makes spare parts for MIXET brand faucets. In the USA, Brasscraft is widely available at “big box” home-improvement stores. Your locally owned, independent hardware store may carry compatible parts too.

There are tutorials available online for this brand. Start here:

• Although the Mixet company is no longer in business, shower valves made by the company are easy to repair by just replacing the cartridge.

https://www.thespruce.com/replace-a-mixet-shower-cartridge-2718990

And

• “How to repair MIXET tub/shower valve”

https://m.youtube.com/watch?v=iQuq3MRlzlM

1) Cut the water off
2) Remove Phillip screw in knob
3) Remove the knob
4) Remove the Lever
5) Unscrew or pull off the valve stem cover
The valve stem is now exposed.
Go to YouTube and you will see several videos on how to remove the stem and replace the O-ring and various washers.
Also do a google on "fixing bathtub leaking faucet", Read the articles and view several videos to become familiar with the terms and parts to replace.