I've read numerous threads/suggestions of how to apparently flatten warped records. - From putting them on the bottom of a stack to oven heating in a 'press' ... .
Would you put a valuable record in the oven??
I think it is almost impossible to flatten a warped record completely in a controlled way. However, I found a way that partially works with the most severe warps without destroying the subject.
If you apply warm (not hot) water to a record (or immerse it) it will warp/bend temporarely and then immediately return to its former shape. If you are quick enough and checked beforehand where the warp is you can apply hand/finger pressure and correct it.
Safer than an oven.