version 4.2.0 and update of installs - Opencart upgrade is available as listed under WHM, but does not allow individual site to upgrade to 1.5.5.1 from 1.5.4.1 ? ?? No "Upgrade" avaiable, does not display for domain via cpanel interface for softaculous of OpenCart.
Sir we have not provided upgrade to the latest version i.e. 1.5.5.1 because there are some bugs (from OpenCart side) after upgrade from 1.5.4.1 to 1.5.5.1 which might cause your site to break. Hence the upgrade is not yet available. Once the OpenCart team fixes this issue we will restore the upgrade capability.