ESPRESSO is a high resolution, super stable fiber-fed echelle spectrograph, installed at the ESO-VLT Combined Coudé Laboratory of the Paranal Observatory. The main instrument subsystems are: Coudé Path, Fiber Link (FL) and Spectrograph. After installation and verification, the FL subsystem flux efficiency did not fully meet the requirements. We describe the FL subsystem upgrade, required in order to improve the optical efficiency and to meet the throughput specifications. We present the opto-mechanical design improvements, aimed to ensure proper reliability and stability of the FL subsystem both in the alignment phase and in the integration on the spectrograph. Moreover, we summarize the activities done for the different units along the upgrade phases: AIT in laboratory, FL subsystem integration on the spectrograph at VLT and on-site efficiency verifications. Measurements done before and during instrument re- commissioning show a throughput improvement of more than 30%.