I guess its made that way so people buy versions contaiting previous stages too.
Anyways, i found a way to enable stage 1 in the latest release, this is what you need to do:
1. Get yourself a .dll decompiler, i used dnSpy
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2. Open the folder where the game is located and go to
MonstersAbyss2.5\MONSTERS ABYSS Ver0.21017_Data\Managed and locate a file named Assembly-CSharp.dll
3. Open the assembly.dll file in DnSpy, i recommend making a copy of the original file just in case.
4. Open the explorer like this:
View attachment 3908955
5. Scroll down the list and search for "stageUIcanvas"
6. Once there, change the code as seen in this image:
View attachment 3908961
Not sure if stage 1 and stage 2 object need to be set to active(true) for this to work, but i'd set them regardless.
7. Save the changes and start the game.
8. If done correctly, you should have five buttons on this screen:
View attachment 3908968
Congratulations, you can now play both stages in a single executable.