How to connect external antenna

I am using the original GSM Shield with integrated antenna (antenova). The signal strength is pretty poor and I need to connect an external antenna. How should I proceed?

Do I just mount the SMA connector or is it necessary to desolder the old antenna? And in that case, is there any other detail to consider, such as the impedance of each antenna?

I’m attaching the schemaitc of the antennae, which I am not understanding. ANY comment regarding this topic is welcome!
Thanks in advance.

I’d suggest asking this in the Electronics section near the top of the board - you’re likely to get more responses up there.