if you're talking about weapons that
when you choose a class below has a 3 cell where weapons drawn in), press and choose the gun you want
if about a suit, a bit of a chosen color, appearance and left (working with a translator to ignore errors))

You select colours or choose the skin for the item you want from the customization screen. There are 3 factions to customize; Agatha, Mason, and Free for All. Changes are saved when you leave that screen.

Once you enter a regular match, you must also select the faction, class and weapon to see any changes.