On the Connected accounts page, you will find an Edit button to the right of each account. When you click on this button, you see the options to Update and Re-sync.
Update allows you to re-authenticate the connection for the account by entering the credentials. This is useful if you just changed your password on the booking channel.
Re-sync will re-import the listings detected in the connected account. Our system automatically re-syncs every 2 hours, but if there have been changes in any of your listings and you want this reflected immediately in Properties, you will want to manually force the re-sync. This will not resync calendar data.