If you have a wallet.dat file, then you will need to download the QT Wallet from here: https://github.com/stratisproject/stratisX/releases. Once you've done this, navigate to the Stratis folder, find the wallet.dat file in that folder, delete it and replace it with the backup "wallet.dat" file you have. Make sure it is called "wallet.dat".

Open the QT Wallet and give it time to sync fully (may take 10 hours or so, depending on the speed of your internet). Once it has, you can then migrate to the new wallet, the Stratis Core wallet, which can be found here: https://stratisplatform.com/2018/12/12/stratis-core-production-release/. There's also a migration guide on that page as well, if you want extra help. To do this, you have to create a new wallet in Stratis Core, get a new address and send the Strat from QT to the new address in a transaction.

There is currently no way of migrating to Core directly using the wallet.dat file, as the architecture of the wallets are quite different (Stratis Core is an HD wallet).