Mirrors googleAds:mutate on the linked CampaignBudget's amount_micros. amountMicros is MICRO-UNITS of the account currency: 10000000 means 10 units, not 10 million.

• A budget is a separate object from the campaign and CAN BE SHARED across campaigns, so changing one may affect more than the campaign you started from; read campaign.campaignBudget first.
• Write — changes a live ad account immediately; needs a write-scoped token.