Advanced higher frenchie here! I got an A at higher and honestly found it a breeze (then again I'm one of those people who's naturally got a touch for languages) my primary advice is focus on your weakest points. For me this was writing and listening. I would learn phrases and re write them over and over again and this technique seems to be working again this time around. I have tons of sticky notes with phrases dotted around my room and I also fill notebooks with redrafted writings. As for listening practice is the answer. My advice for that would be to ask for a spare sheet of paper to take notes on and if you hear a word that stumps you write it down and try to find it in the dictionary during the break periods where you can "study the questions". You may pick up a lot of marks that way! Same for reading and translation. Translation is often the one people struggle most with and my advice would be to simply read it over time after time (if you have time) and make sure it makes sense in English.
I'll give you a heads up though, I've got a feeling the exam might be easier for higher this year because I sat higher Italian last week (I did a crash higher because again I really, really like languages and wanted to learn a third) and everyone agreed that the paper was a breeze. The translation was three lines long! Three! At higher! It was an absolute gift of a paper and the writings and reading were actually on subjects we'd studied (for Italian a lot of readings focus on Italian culture which can go over your head a bit if you've never been to Italy). My prediction for the French higher based on my experience with another higher language this year is that the exam may well be a bit easier than we are expecting. I don't want to jinx anything but I'm hoping that might relax you a little and ease your nerves. Fingers crossed its a similarly decent paper! Don't take your foot off the gas completely with revision as my mother would say but keep revising and try to relax a little.
As for the day before the exam just try and rest. Get a good nights sleep, have a warm bath the night before, have a decent breakfast that morning (banana on toast is my go to) and remember to drink plenty of water and avoid caffeine. Good luck Hun!