IF (make sure you check on this!) the cyan update has been released for your variant of the 520 AND you are on DP, you have two options:
1. Backup everything and downgrade to WP8, then update to cyan, then update to DP 8.1.1
2. Wait for MS to get their finger out of their a** and let them fix the issue where those on DP can get the cyan update too (god only knows how long this is gonna be).