As the fun progresses, you will gain access to further upgrades that you can install within your equipment. In this chapter of our guide to Call of Duty: Mobile you will find information on how to modify your equipment.
Before the start of the competition, you can enter the equipment selection panel. You will find it under the "Loadout" button. There you will see what equipment you currently have chosen. You can change it by clicking on the item and a list of available substitutes will appear. Of course, not all of them will be available immediately. Some of them will be unlocked as the game progresses.
However, if you want to improve your equipment, you need to look under its frame. There you will find several small slots. Each of them symbolizes one available slot for modification. Click on them to move to the next panel. There, you will find a list of all available improvements. Choosing one of them will pop-up information about how it affects the statistics of the equipment . If you are determined to make a choice, click "Equip"  to confirm your choice.
New modifications will be unlocked over time as the weapon progresses to a higher level of experience.