Open the wallet and select "Backup" from the lefthand panel.
A warning will display informing you that only addresses generated by the wallet's seed will be backed up. You can verify whether an address belongs to the seed by opening the Address Book and checking that there is a silver star next to the address. A star that is empty does not belong to the seed and will not be backed up.
Once you have read the warning, click continue. You will see you see further instructions instructions on how to store the seed phrase.
Click continue again, then copy down your seed words. Click the checkbox to confirm that has been done.
Click next, then fill in the seed again.