upgrading 6.0 to the Latest 7.X.X series

Hi there, can anyone help on the upgrading themes issue?

I cannot find any definitive instructions on properly UPGRADING, the responsive themes.
It seems I can only import the new theme and activate, thus losing ALL pages, listings, memberships, in fact just about everything. I then need to start all over again.

Can anyone tell me: Is it possible to upgrade without losing everything? and if so how?

I am currently running version 6.0 and would like to run the latest version.

Many thanks in advance.