You don't shift a bike through the gears in numbered order like a car.

Think of it more like a 4 wheel drive truck. You have "high" and "low" range (corresponding to the big and small front gears on the bike -- on some bikes I guess you also get a "medium" range), and your regular 6 (or 8 or 10, whatever) gears in the back.

You pick the appropriate range (front gear) for the terrain that you're on and then just shift up and down through the back gears.

When the terrain changes significantly (like you start going up a mountain) then you reconsider your "range" and pick a new front gear.