First you need to open warhogs.exe with a HEX editor of your choice. If you use the fixed exe for Windows 10 from my video then the adresses will be the same as mine. If you use another version of the game, just scroll to approximately the same address and look out for the big warning text about DirectX, the part you want to edit is directly underneath it.
The values always go in pairs, the first value is the rank and the value just after it is the amount of promotion points necessary to promote. In the reference section of the wiki you can find a list of the rank values: Ranks
As you can see the values for Ace ("0F" in orange) and Legend ("10" in red) are both "00". This means that you can't promote to them. You can change those numbers to whatever you like (for example "0F 01" and "10 01") to be able to promote ace & legends.
All thanks go to Dr. Sticky Games who figured this out, check him out here: