On Mac

1. Open Parallels
2. Open ControlCenter
3. Goto settings for VM
4. Goto ‘Hardware’ tab
5. Click on ‘Hard Disk’ from list
6. Click ‘Advanced…’ button
7. Click ‘Properties…’ button
8. Drag slider to the new disk size
9. Start Ubuntu VM

Inside VM

1. Find and open the ‘Disks’ application
2. Click on the partition to expand
3. Click the settings / cog icon
4. Click ‘Resize…’ from menu
5. Drag slider to new partition size
6. Click ‘Resize’

That’s it!