When you choose Increase Budget or Decrease Budget, you set the change Amount as a percentage (%) or in dollars (US$). You can also set a limit: for increases, the Maximum daily budget limit stops the budget from going above that ceiling. For decreases, the Minimum daily budget limit stops it from dropping below that floor. As a safeguard, a decrease never takes the budget below US$1. There's also the Set Fixed Budget action, which replaces the current budget with an exact amount in dollars. The change applies to the campaign's or ad set's daily or lifetime budget, depending on what's configured in Meta.