Open up your settings and scroll down to User and Back up
Then open up accounts
Click on your Samsung account (it will ask for your password)
Then scroll down to "back up" and tap it
All of mine were already checked. When I got my s5, as soon as I started it up and signed in to samsung it gives you the option to restore sms and mms texts. I didn't restore mms because I knew I had tons and didn't need them or want to wait. Hope this is of some help.
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