What is HUP Cancelling? And what are other ways to land, I've always had trouble with that
A mindgame trick, really. DAir has a huge autocancel window (where using a move but only getting normal landing lag) at the start of the move. If done correctly, Kirby will yell "HUP!" like he does at the beginning of a DAir, but no hitboxes come out and you can act immediately after hitting the ground.
You perform it by pressing the C stick down for a DAir and immediately fastfalling. For people who don't know about this (it can mess up people who do know too!) the HUP is a huge indicator that "Kirby will attack", so people put up their shield. Because of this, you can get a free grab or inhale because the opponent fell for your mindgame.
As for landing options, there's several:
The direct approach (beat out the opponent with a DAir) useful at times, but often loses to disjoints like, say, Meta Knight's UpB.
The direct approach part 2 (Stone)
Also useful at times as stone beats pretty much anything, but more punishable if whiffed.
The direct approach part 3 (Inhale)
This is for the opponent on the ground who expects you to DAir/Stone their shield. Use sparingly.
The indirect approach (airdodges)
If the opponent is threatening you with massive disjoints, airdodging is a solid option to weave through their offense.
The indirect approach 2 (baiting)
Remember you have multiple jumps. You can bait attacks from below, weave away, and then fastfall through their offense.
Lastly, you can go for the ledge or just away from the opponent. The ledge provides invincibility so it's nice, but it can be rough to get stage control against certain characters.
Those are your main options.