Hi Sudhakar P •,
Welcome to Microsoft Q&A forum.
As I understand, you want to increase/scale your existing Azure SQL Single Database from Standard S2 to S3 and want to know the downtime.
The estimated latency to change the service tier, scale the compute size of a single database or elastic pool, move a database in/out of an elastic pool, or move a database between elastic pools is parameterized as follows:
Please refer to the official documentation here:
https://learn.microsoft.com/en-us/azure/azure-sql/database/single-database-scale?view=azuresql&source=recommendations&tabs=azure-portal and refer to the details like Impact, Latency, Monitor Scaling, Permissions and Additional Considerations before performing the operation.
Let us know if you face any issue or have queries.
Thanks!