Nothing, just tap the three dots on wrongly recommended videos and then "not interested" or "don't recommend channel". The recommendation algorithm might need more than one data point but it will catch on eventually.
The only reliable way to get it to stop recommending inappropriate content is to delete the videos that triggered those recommendations from your viewing history.
Personally, Youtube seems to only recommend me mainstream media now (CNN, MSNBC, even The Daily Show). It's better than pushing me towards right wing independent media like Sargon and Ben Shapiro, which is what it used to do every chance it got, but I very much don't want to watch either option. As a Bernie Sanders supporter, I'm not exactly wanting what either of them are pushing. It's not that I want an echochamber, but it'd be nice to have political commentary which did not try to either obviously lie, or selectively tell the truth in order to paint a false narrative.
Example: I'm a former rock climber and am very interested in knot and knot-tying. I watched some vids on hemp ropes, including "braiding" strands of rope. That points me to a video on hair braiding, which I watch as I do find it mildly interesting, a knot system that won't bind into a mess. Now youtube is recommending me very odd videos of little girls doing makeup and gymnastics. And all the static ads are suddenly only dating aps, "find flirty women in your area" junk.
What do I need to watch to get out of this inappropriate category?