Hello SAJAN SAJI.
Thank you for contacting the Microsoft Community Q&A Support,
I suggest raising a support ticket to the Billing Team to support your subscription upgrade. The Team will be able to resolve the issue you are currently facing.
Click on the link below to raise a ticket
NB: Azure Billing and Subscription Management support is included in the Basic Support Plan without charge. Follow this link to get more information. https://azure.microsoft.com/en-us/support/plans/
Let me know if further assistance is needed.