The build no. of the Android 5.1 update is 222.26.7.ghost_att.att.en.us, and it’s already hit the distribution phase for AT&T Moto X1, model no. XT1058. Several users have received it as back as 6 days ago, while AT&T and Motorola acknowledge the update today.
It remains to be seen whether those who had previously updated to Android 5.1 update would receive a patch or something to new update too, and when if tat’s the case.
Meanwhile, if you are still languishing on Android 4.4.4, not is the time to go for an update.
Hit up Settings > About phone > System updates, and check for an update now. Chances are more than not that you’ll receive the Android 5.1 update OTA in an instant. Even if not, be assured that your turn will definitely come under gradual rollout.