If you have used the IOTA GUI Wallet before...

If you already used a GUI version on your computer you must remove every old .iri file before installing the new GUI.

Simply navigate to the installation path and delete all of the files there.

Windows: C:\Users\Username\AppData\Local\Programs\IOTA Wallet

OSX: ~/.Library/Application Support/IOTA Wallet

Linux: ~/.config/IOTA Wallet