Hello,
HikaSerial Subscription 2.1.0 do not propose that feature.
It's something we have in our TODO list for a future release but in the current available version ; there is no "upgrade".
Even if there is some translation and code about it, that feature is no completed some it's not available. We still need to have enough feedback to have a better vision on which kind of interface and upgrade capabilities we should provide.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.