Combat is third-person only, so if you press the attack button, you'll switch to third-person view.

However, iirc, there's a mod to enable first-person combat. The catch is that you won't be able to see your weapons in that view because first-person models weren't shipped with the game.

Edit: The files needed to enable it seem to be installed with (at least some versions of) the UP actually. Go to your game folder, then copy all files from "Tools/first-person-melee" to "Unofficial_Patch/vdata/items".