Examine the remaining finish
Check the remaining material to make sure it's clear and particle-free. If the polyurethane has crud in it, try filtering it through cheesecloth. Then try brushing it on scrap wood. If it goes on fine, use it.
If the poly is water-based and too thick, thin it by adding water (no more than 10 percent). You can also thin solvent-based poly by adding up to 10 percent mineral spirits, but only if you live in a state that allows it. Some East Coast states and California prohibit users from adding solvents after the can is opened. In that case, you'll have to recycle the old stuff and buy a fresh can.
Always test thinned polyurethane on scrap wood before applying it to your project. Chances are it will work just fine. But if it doesn't look right, buy fresh material.