We were unable to create ingame tutorials within the timeframe, so we are posting them here instead.
Throughout the game, you have to collect energy cells, collect 3 and your max flight height will increase.
Pressing E next to the hover turtle (the hovercraft) allows you to open your inventory to upgrade the turtle, if you jump next to it, you can hop into it.
You can fly anywhere from the ground to a max height determined by your cells.
You can always jump out of your vehicle however and walk, regardless of height.
Cells are scattered across the island in shrines which you have to find, and with npc's who will give you them in exchange for completing a quest.