I have two roku devices but unfortunately my girlfriend set one up to link to her email.
how do I unlink the device from her account?
There's no way to just switch from one Roku account to another. A factory reset clears everything from the Roku's memory and allows you to set it up to a different Roku account.
Settings > System > Advanced system settings > Factory reset
IMPORTANT! When linking your device you may be directed to go to roku.com/link via a computer or mobile device. Type this in very carefully and verify that you actually do go there. Otherwise you can end up at a scammer's fake Roku site where they may request you to pay bogus charges and will collect your payment and Roku account information to use as they wish. If you end up at ANY site not on roku.com (other than the https://my.roku.com/link that you'll get redirected to), back out and try again.
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