Hi Revathi ,
first you have to buy an Azure Reservation: https://learn.microsoft.com/en-us/azure/cost-management-billing/reservations/prepare-buy-reservation?toc=/azure/cost-management-billing/reservations/toc.json
For instance for an Azure Virtual Machine: https://learn.microsoft.com/en-us/azure/virtual-machines/prepay-reserved-vm-instances?toc=/azure/cost-management-billing/reservations/toc.json
For instance for an Database: https://learn.microsoft.com/en-us/azure/cost-management-billing/reservations/understand-reservation-charges-mysql
I
f the Azure Reservation is bought and active the Azure Reserved instance is applied to the VM Resource that fits the Reserved Instance automatically without any downtime.
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Regards
Deepanshu