If there the day before, do the stadium tour. You can go into the United museum with that tour and spend some time learning about the club and it's history. The Munich disaster was a dark time for the club. The trophy room is impressive.
Consider learning a few of the songs, they are quick and easy. It's fun to belt them out at the top of your lungs with 70k+ people. 1, 5,6, and 7 should do. I like 16.
https://www.google.com/amp/s/www.goal.com/en-us/amp/news/manchester-united-chants-lyrics-videos-most-popular-old-trafford-/6gg2ihh94e11anhtzmntwscuManager's name is pronounced "soul-shy-er"
No beer in the stands, if you drink, you have to do so below in the concourse area.
Not part of the United experience, but the National Football museum is in Manchester as well and is a neat look at the history of Football in England. Beautiful church sits next door. Good walking area but not near Old Trafford.
I've been to a lot of sporting events, but nothing compares to United scoring a goal. The place absolutely erupts. Enjoy it!!