Fix hands in images made with Stable Diffusion using... well more Stable Diffusion :p. 

Fix Hands Stable Diffusion Tool

Advanced

More denoising will be more change in the output image. 0.75 is medium. At 0, the output will be exactly the same as the input.

Higher context-free guidance scale will adhere to prompt more

Higher values be more cohesive with the entire image.

Draw in white above the parts you want to change.

Higher values may have more blended edges

Create a few so you can cherry-pick the best one.

Tip: adding details, like 'Highly detailed hands holding a tennis racket' helps.