Hi there! Basically just use File - Open game to load a new one that will replace the old one. The next version will use Tabs to allow you to keep multiple maps loaded (and even used) at the same time.
For the beta the hardcoded limit is 250x250 tiles if I remember correctly, but this limit was mainly implemented into the beta to minimize user crash reports cause of RAM problems. Though we could probably modify or lift the limit for the final version, for the vast majority of cases a 250x250 tile map should be sufficient; especially once we have map tabs implemented.
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Basically just use File - Open game to load a new one that will replace the old one. The next version will use Tabs to allow you to keep multiple maps loaded (and even used) at the same time.
Keep going, it's a really good job