How do I change the data in the previous years' comparison rows on my Data Sheet?

This data can be copied from any other scenario. And you can rename these rows.

To copy data into these previous years in your scenario, go to Data/Formulas > Copy Data, and choose the Copy Multiple Departments option.

This will allow you to copy all data from one scenario into any year in another. Make sure you choose the correct target year. For instance, if you are copying data from Actual 2019 into Budget 2020, you will want to choose Budget 2020 as your target, but select 2019 as the year.

If you are copying data that isn't from a previous year e.g. you have a limit above which your users are not allowed to go in their budget, then you can copy that data across to the previous year, then change the display name of that year.

To change the name, go to Scenarios > Manage Scenarios, click the edit tool on the right of the target scenario, and at the bottom of the edit page you will see options for Alias LY (previous year), and Alias LLY (the year before that).

If you click the Custom option you'll be able to enter a 5 character name for your data, which will appear in that year's row on the data sheet. This won't change the year in the year dropdown menu at the top of the data sheet however.