Instructions
Visit your local air rifle store. Alternatively, find an online retailer.
Purchase an SRC M16A3 upgrade. You can purchase a better scope or a larger magazine for better firing capacity. You can also upgrade other parts of the air rifle depending on the options offered by the retailer in question.
Install the upgrade. Follow the instructions accompanying the upgrade to properly install it. Generally, you need only remove the original piece and install the new one manually. Occasionally, you may need to use some tools to complete your specific upgrade.