Hi Sanket,
Thank you for reaching out. If you are looking to update the Account Budget using Adwords API you would need to use the BudgetOrderService in the Adwords API. Mutate requests to this service are available only to allowlisted Google Ads manager accounts that have also been setup for monthly invoicing. Get requests are open to everyone. You could refer to this guide for more information.
Thanks and regards,
Xiaoming, Google Ads API Team