Sand it.

It will need to be sanded, will bring it up like new

Vacumn the crap out first and sand it. Dont use an aggressive grit though or you'll spend more time getting rid of score marks. I use 180 grit and that seems a good compromise. Wear a dust mask too!

Those kind of bottoms never get really clean because they're textured. I'd rather rip it out, paint up a piece of wood and reinstall. But that takes a lot more time, but it sure looks a whole lot nicer.

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I recently replaced the bottoms of two cabinets. The hardest part for me was getting the nice square cuts. I have to say that it was a huge upgrade because once you get it glued and tacked in, it makes it feel like you’re working on a new box. So much of the yuck and smell is in that bottom board that a new one really freshens stuff up.

It was an easy decision for me because the cabs themselves also needed some significant re-gluing to get them structurally sound. They came out great, compared to what I started with.

Dave

Hand sanding works, especially in the corners. I would not replace the bottom unless is is swollen.

Also there are lots of small palm sanders that can be purchased.

Yeh, this is an instance where hand sanding works the best. Then afterwards, a good sweep with the vacuum cleaner.

Vacuum, palm sander, and by hand in the little corners/ You'll be surprised how fast that crap comes out.

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What about sanding
Leaf blower
And spray or paint the inside of the cabinet