How to back up my wallet using monero core gui beta? I use the windows 64 bits version thank you.
The seed is all you need to restore the wallet. Write it down on a piece of paper. If you want to backup the full wallet history (i.e which addresses you have sent transactions to, and the tx key for each transaction) you need to backup the wallet files. They are usualy located in Documents/Monero on windows, if you didn't choose another location when you created the wallet.
Go to settings on the left-hand side. Then click on the third menu item on the top called "Show seed & keys". It will ask for your wallet password and then give you all you need to restore your wallet in the future. See below image for help.