See any selection of months on the PL by Year/Period Dashboard.
On your Dashboard, under the green dropdown menu, you'll see an option for PL by Year/Period.
This view will default to show a 12 month view based on your fiscal year, but there is Period dropdown menu which allows you to select up to 5 years worth of data in the future, and 4 in the past. Use this menu to select the periods you wish to view.
If you don't see any data in the future months, that may be because you haven't yet added data there. Maybe you are looking at Budget 2020, and you have created the next year's budget in Budget 2021. If this is the case, you'll need to copy data from Budget 2021 into the future year in Budget 2020 to see the rolling 12 months. You can find details on copying here.