Change a Meta ad set budget

Changes the daily or lifetime budget of one ad set. The preview shows the current and new amounts as decimals in the account currency and the percentage change, so the user approves a real number rather than a raw platform value. Use when: the user has decided to scale an ad set up or down and named the ad set and the amount. Do not use on an ad set whose campaign holds the budget — campaign budget optimisation means Meta manages ad set budgets and this call will be rejected. Change the campaign instead. Do not change the budget of an ad set that is still learning unless the user accepts that learning restarts: a large change resets what Meta has learned and delivery gets worse before it gets better. Returns the ad set read back from Meta after the change so the new budget is confirmed, not assumed. Amounts are decimals in the account currency. For currencies Meta treats as whole units (yen, won, forint and similar) the conversion is handled here — never send minor units.