I would deflate the tire, remove the valve core, and dip a dipstick into the valve to see how much (if any) liquid sealant is remaining in the tire. If it hasn't been topped off in a year, I'm guessing it'll be pretty dry in there. If so, add sealant through the valve, using either the Stan's drip bottle or a plastic syringe. You want a couple of ounces of liquid in the tire. Once dispensed, replace the valve core, tighten it up, and inflate. Spin and bounce the tire a bit to distribute the sealant, and you're good to go.
Here's a video of the process, if that isn't clear.