When you are using FastStats to generate a local market update, there is a drop down menu in the upper left that allows you to select your geographic area and the time period:
The SmartMLS FastStats site is very new, so there will not be a lot of historical data/options available right away. As time goes by, we will start to be able to access more previous months.
The previous month's data takes some time to aggregate and is not available in the system until the 15th day of the current month. For example, May 2019's data will not appear in the Time Period drop down list until June 15, 2019.
When viewing a local market update report, the time period of the data should appear in the bottom right: