Guided upright is available under the Transform tab in Develop module.
You could press "Shift+T" to enable Guided upright.
For more info refer Adobe Announces Guided Upright in Lightroom CC « Julieanne Kost's Blog
Guided Upright is ONLY available in the CC2015.6 version of Lightroom, NOT the stand-along 6.6 version.
I am sorry about the misread.
BobSomrak is right.This feature is only available with version CC2015 (not in standalone)