You can unlock New Game+ by beating the game. But if you already beat the game before de 1.4 update, you can either finish the game again or enable it manually.

Steps to do so: 

1. Put on your hacker hat IRL (Mandatory! Send me a a picture!)¹

2. Locate your save files folder: - For linux is ~/.local/share/godot/app_userdata/Trapers Platformer/ - For windows is %USERPROFILE%/AppData/Roaming/godot/app_userdata/Trapers Platformer

3. Create a new file and name it savegame.unlockables.save (file extension is important too!) 

4. Put the following contents in it: 

{"new_game_plus":true} 

5. Save the file and start a new game plus!

Notes

¹: Not really mandatory!