How do you get the handle off to replace the washer on a delta faucet?

In nearly every modern faucet, there is a trim cap on the top center of the knob that is a friction fit. You should be able to get a fingernail under that trim cap and pop it off. Almost certainly you will find a Philips screw under it; remove that screw and the knob should then come off. You may need to apply some force to the knob; crud getting between knob and shaft, carried there by water, will tighten the fit over the years.