You will edit your Office 365/Sharepoint Access Web App on the desktop using a file type of .accdb
Backing up that file is a waste of time. It just provides pointers to all the data and objects that are stored in Sharepoint/SqlServer.
Understand What I am About To Tell You
The backup is a snapshot of your objects, table structure and maybe your data at a given point in time. It is important that you use a date in the file name Depending on the relationships and data you might end up deleting data from the live database.
Here is how you backup your Access Web App
Figure 1 - Go to the file menu
Figure 2 - Save the package
Figure 3 - When saving the package, add a date to the file name and click Data if you want to save your data
Now You Can Start to Recover the Database
Figure 4 - In SharePoint, you need to add an App, Find Access Web App and upload file
Garry's Golden Rule of Backups
It doesn't matter how good your backups are, its the recovery that matters. So once you have made an App (say 20140503 AWA.app), restore it to a SharePoint/365 site and make sure that it works.
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