The Accuracy Upgrade allows the player to increase his Accuracy Stat by 10. You are required to give the voucher to Feris to receive the upgrade. You are given one by Macil each time you complete a mission.
Stamina and Accuracy Upgrades can also be given to the player automatically by completing certain objectives in the game, such as completing the Training Facility [MAP31], disabling the synaptic jammer in The Mines [MAP14], destroying the Autonotomic Converter in the Conversion_Chapel [MAP24], and killing the Loremaster in The Lab [MAP27].
To see the effect of the accuracy upgrade, you want to use the Targeter.