First of all please check article http://prestabay.involic.com/prestabay-startup-version-upgrade-1-6-0/
If you interested in this upgrade you can purchase it by the link in article attachment. Before installation please backup your DB and save copy of files at /module/prestabay/
For do upgrade, please check article http://prestabay.involic.com/HowToUpgrade

Also, upgrade from 1.5.x to 1.6.x version required re-assign all ebay accounts and re-download marketplace information.
After upgrade installed, please open page Category->Ebay->Configuration and click on the link "Get License". This will generate for your ten days trial License. 
After we review your order trial license will be converted to full version.