Cleaning pressure cookers advice
Well to clean the pressure cooker,Immediately after use, fill the pressure cooker with warm water and add a small amount of washing up liquid, and allow it to soak for an hour or two hours. This prevents any cooked food from becoming embedded around the inner regions of the cooker .
Then wash the pressure cooker by running it under hot water and using a cloth or a sponge and apply some washing up liquid cleaner. Rinse the cooker and dry it, using a dishcloth, to ensure that no excess water is left that could cause water stains.This way you can clean the cooker after daily use.

But to remove permanent stains you can use cream of tartar which is a spice along with lemon juice.
To that boil one litre of water in a container by adding 3 tablespoon of cream of tartar and 3 tablespoon of lemon juice. And pour this solution to the pressure cooker and allow it to stand for an hour or two.

Then using a plastic scouring pad or sponge clean the pressure cooker, avoid the metal pad as it might damage the cooker.

Hope it might help you and enjoy cooking.