First, remember the unlock progression of the item you want to unlock. This works best if you’ve already made some progress towards a cosmetic you want to have. I think it should also be possible to use this method for cosmetics you have no progress in whatsoever, but I have no experience in that regard. Ergo, that won't be covered here. For this guide, I’m going to use the Lab Coat as an example, which is unlockable by leveling up 100 times as the Scientist. Before doing this, I already leveled up 23 times for the requirement.

To start off, convert the current level progress towards the cosmetic you want to hexadecimal. You can use Google for this. Just look up “Convert x to hexadecimal”, and it’ll show up. Because I already leveled up 23 times as the Scientist, I converted that number to hexadecimal, which is 0x17. Remember the converted hex value well.

After that, you’re going to back up your progress.data just in case something goes wrong. Just copy it and put the back-up in a secure place. Editing this file is easy, but it's better to be safe than sorry.

Then, take the progress.data file from your Pulsar: Lost Colony folder and open it with a hex editor. This can either be a downloadable program, or even a browser-based editor. I used hexed.it to do it, which is a web-based hexadecimal editor. You can't directly edit the progress.data file using Notepad, because it ends up looking like gibberish when opened.

Open your progress.data file in hexed.it (or your hexadecimal editor of choice), and look for the hex value that corresponds to the progress you've made towards the cosmetic you want to unlock. Hexed.it has a search bar on the right side of the screen that makes finding the value you need to change easy. For this example, you need to level up 100 times total to unlock the Lab Coat. Convert 100 to hexadecimal, which is 0x64. Replace the 0x17, which represents the current progress towards unlocking the item with 0x64.
I’m no IT guy, but you don’t need to replace the “0x” part to unlock things in progress.data. This part probably sounds stupid to someone who has actual experience, but this was my first time ever fiddling with hex values so I thought I'd share my experience with it.

After that’s done, save the file and replace your current progress.data with the file you just edited. Start up the game and ta-da! You should get the notification that you have unlocked a new cosmetic item. You can even share this file with your friends, which allows them to use the cosmetics you've unlocked.

I hope this guide served you well. Feel free to ask questions down below, and I'll try to answer them as best as I can.