Switch backup policy for a VM

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