Once you have re-exposed the valve, unscrew it from the sink. It’s very small and should come right out. If the threads where the valve screwed into the sink area are dirty or rusted, sand them. On the base of the valve is a small ring of rubber (the gasket) that acts as the seal between the valve and the pipe. If this ring has deteriorated, take it off and take it with you to your local DIY store and get a replacement. They cost pennies. Pick up a tube of plumbers putty while you’re there.
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