Update an existing budget. Only the fields included in the request body are modified; omitted fields remain unchanged.

Common updates include adjusting the target amount, modifying scopes, updating subscriber lists, or changing threshold percentages.

A hierarchical budget's allocations must cover exactly the values of the scope they split, after the update as well as before it. To add or remove a value, send scopes in the same request as hierarchicalAmount, or, for a variable budget, as periods covering every current and future period. An update that leaves them mismatched fails with 400 and changes nothing.