You have two hard disks, Hard disk 1 and Hard disk 2 on a VM.
And you would like to change this order like below.
Before
Hard disk 1 10GB
Hard disk 2 20GB
After
Hard disk 1 20GB
Hard disk 2 10GB
You can change this order by below procedure in VM Edit Settings.
1. "Remove device" Hard disk 1
2. ADD NEW DEVICE as Existing Hard Disk which is removed device before