How to fix leak

John, Easy fix, remove and replace the tub spout with new one equipped with diverter. Your old tub spout will be attached with one of 2 methods, set screw under base of spout, usually an allen/hex type screw which needs to be loosened or the other attachment is spout is threaded onto stub pipe, in this case you will have to turn spout counter clock wise to unscrew and remove.

If diverter is the type found between the shower taps(3rd handle), this diverter handle/knob is remove and the cartridge is removed and replaced. Take cartridge with you to plumbing supply to mach it up.

Check the attached links,instruction and guides, Good luck.

I hope this helped you out, if so let me know by pressing the helpful button.

https://www.thisoldhouse.com/ask-toh/rep...

https://www.youtube.com/watch?v=Pbw1JCf8...

https://www.youtube.com/watch?v=do_ltqVS...