Google for HxD Hex edit. The other I used expired in 2 years....didn even know it was a trial...
After that You need "one click tool" so you can find the outfit you want to mod it.
Note: One click is not updated anymore only the community has been updating it so if it's a newer costume you may not find it.
Also you need to set up the correct paths with it (have fun)
After all that simply youtube some mod tutorials or check the forums videos.
The problem is that i followed the tutorial for one click tool to replace a costume i wear with another one but it doesnt work. I change the path too but i guess i do it wrong. The software say completed but its not replaced