You can find the talent modification menu by tapping the Main Menu, and select Firearm.

Firearm Upgrade:

Use Firearm Components to upgrade the level of firearms and increase their attack, penetration and specialization. Certain levels require additional Carbon Fiber Parts.
Firearm Components and Carbon Fiber Parts can be obtained from the System Shop.

Attachments:
Each firearm has four attachment slots which generally go by the Barrel, Optics, Magazine and Stock combination, albeit named slightly differently for some. Attachments for these slots provide their own skills in addition to base attribute bonuses.
Attachments can be upgraded by combining two of them into one at one level higher. Upgraded attachments provides significantly higher bonus to base attributes and to their attachment skills.