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