It's a tough conversation to have, this was actually a roleplay scenario we had to cover in a management training course, I can't remember everything that was said now but one thing to keep in mind is if this has deteriorated over time it could be symptomatic of some underlying issue like poor mental health, illness etc so I wouldn't go in all guns blazing. One of the key things however was not to not be too indirect i.e. don't just talk about hygiene in an abstract way, make it clear that it's something you'd like them specifically to focus more on but do probe beforehand to understand how they are feeling in general, any problems outside work etc.
I'll be honest, in the past my personal hygiene hasn't always been the best (without being awful) depending on what's going on at home, lack of time etc when I used to commute 20hrs a week, I reckon if someone called me out on it in the right way I'd be a bit sheepish and focus on it a lot, sometimes you just need a polite kick up the backside.