First step is to go to the settings.ini file that is located C/Users/{Your User}/AppData/Local/Sega/SonicMania
The second step is to open it and add under [Game] devMenu=y like in my screenshot.


Now its time for the all medals cheat.
First step will be to press up up, down down, up up up up when the title screen appears, you have the time when sonic appears to right before the press start text. (should hear a ring noise)
The second step is to go in the level select (I used the dev menu)



The third step is to put 01 09 07 09 00 08 01 01 in the sound test. (I forgot if you hear a ring noise or not after doing this)
The last step is to put 01 09 08 09 00 05 00 01 in the sound test. (should hear a medal sound)



I have finished all my saved games with debug mode.

Special thanks to Zoe the Fox and bob.bmp for helping me.