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.