Jairo:
In the "show version images" output you provided, you still have 15.5.3.4 in the primary partition. Simply issue
use image primaryand reboot the switch. I presume this is the image you were running prior to the upgrade to 16.3.1.6.
There's no way to roll back to whatever image existed in the secondary partition prior to your upgrade, but since EXOS does not allow you to install to an active partition, it's likely the primary is what you were running before. Once you're booted into the primary partition, you can download a new image into the secondary partition, using the article detailing software upgrade provided by Henrique above.