Two months ago I finished staining my deck.

I was using:

Benjamin Moore Arborcoat
Premium Exterior Stain
Classic Oil Finish
Deck & Siding
Semi-transparent

Last week I wanted to do some touch up
staining on a small piece of the deck, not even larger than a square
foot and on the side of the deck (not on top of the deck). So I opened
my can of stain. When I finished staining my deck in June I had a little
under a quarter of the can left. But when I opened the can the stain
was now dry. Is there a way to revive this stain since I only need a
very small amount? Should I add a little paint thinner? Or is there a better way?