You can set an annual spending decrease rate (% of how much less they will spend each year. Default 1.0%) under the Retirement Spending Goal, in the Retirement Planner.
Select the pencil icon to edit the Retirement Spending event. Under "Spending Decrease", you can adjust the slider bar to the percentage of your choice. Once saved, it will display, in your Detailed Cash Flow table under Retirement Spending, the amount spent each year with the % applied until Zero or when the Spending Minimum is reached.