1. Download HexEditor for example here [filehippo.com].
2. Go to C:\Users\YOUR USERNAME\Appdata\Local\AliensVsPredator
or
it could be in your documents folder.
3. There will be a folder with numbers. Open it.
4. Inside, you will see one file with ''.prf'' extension. It also will have the same numbers as the folder you just opened.
5. Open that file with HxD (Hex Editor program you downloaded in step 1).
6. Scroll down the offsets until you reach the line ''00000A20''
7. In this line, the columns are corresponding: 5 - to predators; 9 - to aliens; D - to marines.
8. Replace the values with the number of the skin you prefer. Look below for the values:



9. Save the file. Don't make a backup if asked.

Then just play the game!

Note:
* If you reinstall the game, you will have to do this process again. There is no workaround for that.
** To see skins just go to multiplayer menu. They are all unlocked there.

Here are all the skins you can unlock in the game.

https://steamproxy.com/sharedfiles/filedetails/?id=1738960705