Logic Pro 10.5.1 is a historic milestone in the evolution of Apple's Digital Audio Workstation (DAW), marking what many consider the software's most significant update since its 10.0 debut
Getting Started with Logic Pro 10.5.1
The only legitimate way to get Logic Pro is via the Mac App Store.
Pro producers often "zip" their Logic application before updating. If you did this before moving to 10.6 or 11, here is how to revert: System requirements and versions for logic pro