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If you don't mind hex-editing peng-dos.exe, go to file offset 0x4d06d
change 0x85 0xc0 0x74 0xe4 to 0xbe 0x1d 0x00 0x90
then start the game. Now you can select any level.
There is a cheat mode which enables selecting all levels, but I don't know how to enable it.

proper cheat mode looks like pressing something during the credits.
how do you make the credits display?

peterferrie wrote:

If you don't mind hex-editing peng-dos.exe, go to file offset 0x4d06d change 0x85 0xc0 0x74 0xe4 to 0xbe 0x1d 0x00 0x90 then sta […]
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If you don't mind hex-editing peng-dos.exe, go to file offset 0x4d06d
change 0x85 0xc0 0x74 0xe4 to 0xbe 0x1d 0x00 0x90
then start the game. Now you can select any level.
There is a cheat mode which enables selecting all levels, but I don't know how to enable it.

Interesting way how to do it.

Does anyone else know what software I can use to edit the hexes of the EXE?

Google hex editor

Ok, I got Hex Workshop, but I need help finding 0x4d06d, so that I can change 0x85 0xc0 0x74 0xe4 to 0xbe 0x1d 0x00 0x90, like how peterferrie said to do.

you don't have to find it. It's a location. (like 10 miles from your howetown)

Qbix wrote:

you don't have to find it. It's a location. (like 10 miles from your howetown)

actually it would be more than 6 times harder to find than on that 10 miles long road 😉

peterferrie wrote:

proper cheat mode looks like pressing something during the credits.
how do you make the credits display?

I found out the credits appear after beating level 30, but I'm not sure how to activate cheat mode.

yes, I found that, too.
I suspect that it's not a cheat mode after all, but simply that it allows to you play any level once you have completed the game.
if you're good enough to beat the game, then you obviously don't need to cheat. ;-)