the bath tub water spout/spigot has a pull-up knob to get water to the
shower head. it's not closing fully, and about half the water is still
bypassing the shower and ending up in the bathtub.

Is there a fix/repair for this?
or should I go and buy a new spout only? The fixtures are only 10 years
old, so I'm surprised it's failing already...

I'm good at DIY, so I'm willing to fiddle a littlw with this..