That post is very simple to take apart and fix your problem. When you disassemble you'll be able to easily push that seal back in place. Unscrew bottom, two 2mm bolts and its apart. Add some of the grease from the packet they sent with the post if its dry inside when you reassemble.
can.oneupcomponents.com

DROPPER POST V2 CLEAN & GREASE SERVICE
Tools needed Waterproof grease (ie. Slickoleum, Slick Honey) Rubbing alcohol 2 & 5mm hex Pick tool Needle nose pliers Strap wrench 14 mm or 17mm wrench (or adjustable wrench) Lint-free Cloth COMPLETE EXPLODED VIEW & PARTS LIST This service should be completed every 50-100hrs of riding depending...
