Install a new DigiByte wallet and open it up.
Once you have located your existing backup file shut down the new wallet that you just opened.
Make sure you are able to show hidden files on your operating system so you can see the DigiByte data folder.
Navigate to the DigiByte data directory folder. By default this is in C:Users"User"AppDataRoamingDigiByte
There will already be an existing wallet.dat file from the new wallet you just installed. Delete it and replace it with your old wallet.dat backup.
Restart your wallet and let it finish syncing. If your funds do not appear you may have to restart the wallet with the -rescan or -reindex commands.