You also can save multiple island-designs on your pc or have an extta savety-location for your save:
After saving you will find the path:
C:\Users\Besitzer\AppData\LocalLow\bobacupcake\IslandPlanner\saves
Grab a copy of "islandmanager.island" inside there and paste it into a location of your choice.
For multiple saves just have multiple folders there to put one save in each.
To load one of theese into the app, be sure, you made the previous steps with your previuos work, than copy the save of your choice and paste it into the path, written above, overwriting the file there.