If you've played the newer demo (2.X+), you'll be able to transfer your save over to the full game with the in-game convert button!

In the full game, head to the file select menu. If you have a valid demo save you'll see a small 'Convert Demo Save' button next to the empty slot. Click this and the game will convert the save to work with the full game.

If you've completed all you can do in the demo, you'll need to still go back to the final pool of the Spawning Pools (Northern Pool), and use it. This time instead of being greeted by the Pondmother you'll go to the dream world as normal, and so you can cross the bridge to a pressureplate that will let you open the path to the Central Junction. Dendro will meet you across the water!

If you haven't got to that point in the demo you can just continue as normal - if you really haven't got far in the demo then i'd recommend just making a new save!

Any bugs with this functionality please let me know! I've tested it a bunch but there's only one of me <3