How can this be fixed?

Hi Nancy...

Symmons sells a washer kit (first image) to repair this or you could simply purchase an entire stem assembly (part TA-10... 2nd image) and just swap out the entire stem assembly. Parts are easily available at all home improvement stores. See exploded view of your valve below (last image).

You will just need to shut off the water supply, remove the trim, and then unscrew the stem assembly. Swap out the washer or the entire stem assembly and reasemble all. With the symmons valve be sure to leave the stem assembly in the open position so the handle will not bind before final tightening.

You are lucky, because symmons is very easy to repair... most of the time!

Let us know how you make out... OK?

Mark