How to clean the stained insoles of sandals?

your question posted twice here is an answer for you

https://www.youtube.com/watch?v=h6-rbLF_F54

How to Clean Sandal Footbeds – 7 Easy Tricks to Make Your Sandals Look Brand New

https://www.vbmbestreviews.com/how-to-clean-sandal-footbeds/

You can make a scrub of hydrogen peroxide and baking soda into a paste consistency, then use a brush or scrub daddy to clean up. Wipe with a damp microfiber cloth

If the dirt is just manageable you can use a magic eraser of dawn detergent. However if it is bad, you can make use of a cleaner for that. Here's a great resource you can get information from https://www.johnhollanderdpm.com/7-ways-to-clean-and-freshen-up-your-sandals/#:~:text=Wipe%20down%20the%20footbed%20of,allow%20the%20shoes%20to%20dry.

If the dirt is just manageable you can use a magic eraser of dawn detergent. However if it is bad, you can make use of a cleaner for that. Here's a great resource you can get information from https://www.johnhollanderdpm.com/7-ways-to-clean-and-freshen-up-your-sandals/#:~:text=Wipe%20down%20the%20footbed%20of,allow%20the%20shoes%20to%20dry.

I would use dish soap and a rag, maybe even a Mr. Clean Magic Eraser.