I had a someone put down treads on the stairs.They did one wrong.How to I get PL400 construction glue off.

Did you try a chisel or 5-in-1? Maybe just tapping it with a hammer and chisel will break it loose. Let us know what you have tried.

Are you trying to remove the tread that has been glued down? If so, use a hair dryer and warm it up very slowly. The glue warmed glue should allow you to lift the tread off.

Also try mineral spirits.