Step 1 : Create a new policy ( For example : Daily30Monthly90j )
Step 2 : From Protected items and Backup Items, select Azure Virtual Machine.
==> Search for the target VM and click
Step 3 : From the vault item's dashboard ==> select the backup policy ==> change policie and select the new one and then select Save.
The new policy is immediately applied to the vault.
Step 4 : Run an on-demand backup.
==> Select The VM and select Backup Now to create an on-demand backup. (triggering backup launched)
The Backup Now pane appears.
Select OK to run the backup job.
To track the job's progress, on the vault dashboard, select the Backup Jobs tile
Clique on 'View details' to see the progress
Officiel Source : https://docs.microsoft.com/en-us/azure/backup/backup-azure-manage-vms