You can modify a budget, using the alter option.
Go to Gateway of Tally > Accounts Info > Budgets > Alter
In the Budget Alteration screen, modify the fields as required. Change the period or change budgets of Groups, Ledgers and Cost Centres.
Note: By default, Set/Alter Budgets is set to No. Set this option to Yes to alter.
To delete a budget:
Go to Gateway of Tally > Accounts Info > Budgets > Alter
Press Alt+D.