If you have wallet.dat backup file for Bitcoin Core Qt client and want to restore it, do simple procedure:
When you placed the other
wallet.dat file in place, you should run Bitcoin-Qt with the
-rescan option. New versions of client do it automatically. Here's how to do that in Windows using the command prompt:
C:\Program Files (x86)\Bitcoinusing Windows Explorer.
Now Bitcoin-Qt should start and rescan the blockchain to calculate the balances of the addresses in your wallet.dat file.
New version supports several wallets. Just upload different wallet files. Start client and choose menu File->Open Wallet choose one.