You will need to download the tree first. To do this, do the following:
	- 
		On the family you want to back up, click on 'Download/export this family'.
 
	- 
		Select 'FamilyScript'.
 
	- 
		Click 'Download'.
 
	- 
		Save the file which downloads (I recommend in 'My Documents' or equivalent'). Make sure you give it a name you can easily recognise.
 
	- 
		You can now download the file easily onto a penstick, or store it on your computer.
 
	Hope this helps!