i feel your pain! Just recently (and filed under stuff my mom never told me about) I discovered Bar Keeper’s Friend cleaning supplies. They gave a soft scrub which works amazingly! Grim that was many years old is now gone and my pots looks like new!
Steep it overnight in Hot soapy water. Use Brillo Pad on it in the morning.
This video will help: https://www.youtube.com/watch?v=GJwS_MmgdDI
It's pretty easily done but takes a bit of scrubbing.
I always keep SOS pads available for this too.