You need to put the wallet.dat file in the data directory of your installation.
C:\Users\USERNAME\Appdata\Roaming\Bitcoin is the standard path (windows 7/10; if you didn't change it when installing).
You can open this path with pressing
Windows-Key +
R and typing
%APPDATA%\Bitcoin.
Make sure to backup/rename the wallet.dat which currently is inside this directory (e.g. wallet.dat -> wallet.dat.backup).
Then make sure to move a copy of your wallet.dat which you want to decrypt inside this directory.
The next time you will start core you will be able to put in the password to decrypt your file.
Keep in mind that core has to be fully synced to see the latest balance. If you wish to skip the syncing you could export the private keys from core and import them into a lightweight client (e.g. electrum [1])
[1]
https://electrum.org/#home
