Welcome to the Forum @Ajinkya_Mishra and thank you for reaching out!
If you’re the billing owner, you should be able to change your premium plan size directly from your account following these steps: [https://asana.com/guide/help/premium/billing#gl-downgrade]. Please note that any change you set on your Plan will be active at the end of your billing cycle.
In case you need further assistance with this, you can reach out to our support team following these steps: How do I contact support. They will be more than happy to assist you by manually change your plan.
Hope this helps! Please let me know if you have any follow-up question!