A lot of stuff I wouldn't change because it helped make me who I am and opened up possibilities that might have not otherwise happened. However, I would tell my past self to take better care of myself, because now I'm paying for my past negligence. I'd also encourage myself to transition earlier.
I'll tell 'her' romancing or affairs are nice but it's a crap. that she'd seriously better try save up than pouring her energy and time into love affairs so she'll be able to save the ones when she wants to protect them. Alright? Are you listening, Yuki? It's not too late. Try hard.