You can increase the size of the disk from the same Virtual Machine Properties screen that you attached to your post. Then use a partitioning tool like gparted to increase the partition size to use the newly added space on your data disk.
Depending on the layout on RHEL itself you have 2 options:
- The system is LVM based, extending this is very easy and there are quite some guides on the internet
- The system is pure disk based, you can extend the system live with fdisk -l /dev/sdX (where X is the disk) but this might be dangerous. The safest solution is to use a tool like gparted