The file is located in C:\Users\YOURUSERNAME\AppData\Local\AliensVsPredator\76561198054965893
(note: the number at the end may be different for you)

Once you open the file needed, scroll down the offsets until you reach the line '00000A20'
Keep in mind that this line, the columns you see are corresponding:
05: Predators
09: Aliens
0D: Marines

00000A20 x Offset 05 is the Predator skin you have equipped (I have 07 as shown in screenshot)
00000A20 x Offset 09 is the Alien skin you have equipped (I have 05 as shown in screenshot)
00000A20 x Offset 0D is the Marine skin you have equipped (I have 04 as shown in screenshot)

I have highlighted the important numbers for the skins (DO NOT CHANGE ANYTHING ELSE)
Changing the numbers highlighted will change the skin you had equipped.

Remember to save the file and do not make a backup if asked.

Predator skins:

• 00 Dark
  
• 01 Claw
  
• 02 Stalker
  
• 03 Hunter
  
• 04 Spartan
  
• 05 Wolf
  
• 06 Lord
  
• 07 Alien

Alien skins:

• 00 Warrior
  
• 01 Warrior Dome
  
• 02 Number 6
  
• 03 Warrior Ridged
  
• 04 Praetorian
  
• 05 Nethead

Marine skins:

• 00 Franco
  
• 01 Rookie
  
• 02 Van Zandt
  
• 03 Moss
  
• 04 Kaneko
  
• 05 Connor
  
• 06 Gibson
  
• 07 Johnson

If you are having trouble understanding then go to my video and do exactly what the video does.
https://www.youtube.com/watch?v=7tnQ1hQtkmE

Make sure you have your game closed and delete the duplicated file that is created when you save it. Then launch the game after you "change" your skin.

HxD is not a virus or anything. But it can be bad if you don't know what you're doing, which is why you must watch the video carefully without making a mistake.
DO NOT USE HXD ON ANYTHING OTHER THAN THE NECESSARY FILE

note: This process doesn't "unlock" the skins, it just forces the game to equip it for you. If you change your skin you won't be able to re-equip the pre-order ones unless you do the whole process again.