Apple made it easier to resize partitions–logical divisions of a disk drive into separate mountable volumes with different properties–several releases of macOS ago. A Macworld reader resized their main volume to 369GB to set up a Boot Camp partition, but then realized it was too small. They wondered how to fix this.
In the right circumstance, you can simply follow these steps without having to back up the entire drive, erase and reformat it, and add new partitions.
Always back up your drive before attempting to resize partitions in case something goes wrong, or you accidentally click to proceed on a destructive operation.
Read more at Macworld.com