I'm not a builder and my husband is sleeping but I believe the best solution is to replace the stairs. They look like they were made with cheap materials. You will want to talk with an expert as to what kind of wood they recommend.

It won’t cost the earth to replace these so i would do it

It looks like a pre-fab unit designed to be used indoors and covered with carpet. The material is OSB- oriented strand board- which expands when it comes in contact with water. You'll need to replace with a material that can withstand moisture.

It's a rebuild, and it'd be nice to install a landing at the top if you've got the run.

Rip them out and have a new stair built using Pressure Treated lumber or a regular pine set of stairs will last forever...

I don't know why anyone would use this cheap set of stairs built out of particle board in the garage, it will get deteriorated in no time.