Which file does this skin replace?

WEP_1P_9MM_TEX.upk in KFGame\BrewedPC\Packages\Weapons\9MM inside the Killing Floor 2 folder.

Yes. The author warns to copy the stock file elsewhere first, because once replaced the unmodded skin is gone until you restore it.

Mods & Workshop

KF2 9mm Pistol Urban Storm Skin: 2015 Install Notes

V3n0m, the modder behind the AK-12 Vulcan skin, published a second skin in June 2015: the 9mm Pistol "Urban Storm", based on the Dual Elites "Urban Storm" finish from CS:GO. It is Version 1 and the author planned more changes. As with the first skin, it works by overwriting a texture package, since official Workshop skins did not exist yet.

Install notes

Locate your game folder, by default C:\Program Files (x86)\Steam\steamapps\common\killingfloor2.
Go to KFGame\BrewedPC\Packages\Weapons\9MM.
Replace WEP_1P_9MM_TEX.upk with the skin file from the author's download (not reproduced here).

Warning

Copy the original .upk to a safe place before replacing it. If you do not have a backup, use Steam's "Verify integrity of game files" to get the stock file back.

Notes

The 9mm is every perk's starting sidearm, so this skin is visible in every class.
A showcase video was posted by the author on YouTube.
For safer methods in current builds, see weapon skin ini tweaks.

FAQ

Which file does this skin replace?: WEP_1P_9MM_TEX.upk in KFGame\BrewedPC\Packages\Weapons\9MM inside the Killing Floor 2 folder.
Is a backup needed?: Yes. The author warns to copy the stock file elsewhere first, because once replaced the unmodded skin is gone until you restore it.