"resetting" a cdr

Ok say I have a cdr with like 100mb of junk on it and i want to burn an image that is 200mb. Is there any way to “reset” the cdr so it reads as a blank cdr with only the space left as its max size?

Technically no.

If you did not close the disc then you could just burn a 2nd session without importing the first.

If you did close the disc then there is no way to record anymore onto the disc as the contents have been written already & you cant re-write onto a CD-R