You are asking about a desktop product but tagged as online. Confusing at best
Pro Plus 2020 is an annual subscript[tion based desktop product, same as QBDT Pro but as an annual subscription with extra support - but still installed locally on your hard drive under Windows. Once you download and install you simply find your existing DESKTOP (*.qbw) company file and open it.
If you are asking about upgrading from a lesser version of QBO to QBO Plus you go into company settings and upgrade
If you are asking about something else please elaborate
