How to Resize the Boot Camp Partition
If you use the Boot Camp feature on your Mac computer to dual-boot a Windows operating system, then you may find that you need more space. If you have underestimated or overestimated the required space for your partition, you can repartition the drive. Using Mac's "Disk Utility," you can change the size of your partitions. Disk Utility is an application found in the "Utilities" group.
Click the "Applications" folder. Click "Utilities." Click "Disk Utility."
Click your hard drive in the drives list on the left.
Click and drag the corner grabber between the "Macintosh HD" and your other partition. Move it up to allow more space for your other partition. Move it down to reduce the space.
Click "Apply" to apply the changes.