To change the units of the Scribble Maps editor you can do so under UI Settings. You can change the measurement system between metric and imperial. In addition, you can change the unit you want visible for both length and area.
Settings Imperial/Metric & Default Units
In the settings panel select either metric or imperial. You can also select a default unit to use for both length and area with the two drop downs.
Getting to UI Settings
There are two ways to get to settings. The first is through the icon in the bottom left and the second is through the menu.
1. Bottom Left Icon
In the bottom left of the editor there is a gear that will open up the settings dialogue.
2. Through The Menu
Click Menu in the top left and then click the "UI Settings" Icon