i'm on marketpress 2.9.4.2

After the huge redo of the plugin a while back, I'm scared to death to update my version (i'm on marketpress 2.9.4.2 ). I tried once and it broke everything. Any suggestions on the best way to do this without any downtime? I rank high in google and get many visits and I don't want to be down. But this looks like a huge task to undergo..... Advice?