How to restore a backup?

It depends where the backups was created to:

Google Drive
Open Ministry Assistant, go to backup page, restore tab and select Google Drive. Select the account in the list if asked and then select the backup file from the list of backups.

Email
Is the backup is attatched to an email, then open that email on your phone/tablet and open the attachment and select Ministry Assistant.
or
Save the attachment to file, open Ministry Assistant, go to backup page, restore tab and in top right corner use the browse button to find the saved backup file and restore it.

SD card or device memory
Open Ministry Assistant, go to backup page, restore tab and select device memory and restore the file from the list.
or
Open Ministry Assistant, go to backup page, restore tab and in top right corner, select the browse button and find the backup file to restore.