This is very easy, just follow this steps and try to not get confused.
How to Expand Island
- Go to the strange stone pillar (like the one on the image) on your island.
- Buy and expansion (by pressing E) with points (points can be unlocked by destroying trees, rocks, plants, etc).
It shouldn’t be so difficult so you can do this easly
How to Get to Autumn Island
After some expansions, Autumn Island should be unlocked (you can get to Autumn Island by pressing the button on the stone pillar, and you can get back to Aqua Island by pressing the button that is also on the stone pillar).