First of all please check article
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

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.