TL;DR: after 20 hours of training of prospective piano technicians, one instructor estimated about 35% of them were actually capable of consistently doing it to an acceptable level. Another said it took him 6 months as an apprentice before he felt competent. So, yes, you could do it yourself, but it’s not particularly easy. You need a specialised tuner (for complicated reasons, standard equal temperament tuning apps will cause the piano to sound out of tune), and if you use the wrong tools or technique you can cause much more significant damage to the piano.
This was one of the first results that came up for me when I searched.
TL;DR: after 20 hours of training of prospective piano technicians, one instructor estimated about 35% of them were actually capable of consistently doing it to an acceptable level. Another said it took him 6 months as an apprentice before he felt competent. So, yes, you could do it yourself, but it’s not particularly easy. You need a specialised tuner (for complicated reasons, standard equal temperament tuning apps will cause the piano to sound out of tune), and if you use the wrong tools or technique you can cause much more significant damage to the piano.
Surprisingly complicated.