To customize your weapons, simply bring up your menu and go to the inventory.
However, before you can customize your weapons, you need to gather crafting parts, which can be found scattered all over the world. Customization allows you to add muzzle attachments like silencers to your guns, and also improve the performance of your weapons. Customizing your weapons and adding new attachable parts to your gear can really help you out a lot in Deus Ex: Mankind Divided.
