Up-managing is basically supporting your boss. Help them with their goals, find ways to unblock them, ask them how they're doing/let them vent, make them look good to others.
Personally I disagree with the whole "making someone else look good" management philosophy (including "only complain up"). I think it's dishonest and counter-productive. Sure it reduces friction, but it doesn't help crappy things be less crappy. It does prevent people from having hard conversations which might change things for the better. We need to take the ego and resistance to change out of work/management.
Because often the people doing the thing have a pretty good idea what’s possible and what’s not, and when the management wants unicorns they will be unhappy with everyone because all they are getting from them is ponies. Someone has to tell them that’s because unicorns don’t exist and they’re being not just unreasonable but unhinged.
Problem is that if you’re up-managing your boss, they aren’t up-managing their boss. I’m sure there’s a Law in there somewhere. The leader types tend to know what’s what and shield the team from bullshit so they can get busy making the boss look competent in every other area. You can talk your way out of a lot of stuff when people are happy with the rest of your results. Delay tactics if nothing else. Sorry we can’t work on bullshit #372 right now because we’re busy being awesome on feature #784. Come back in a month or two (then cross your fingers to see if they still remember or are focused on bullshit #381).