How to adjust weight in pro-extender

Hi All,
I am new to thunders place. I like this forum very informative on PE.
I am using pro-extender for almost one year(2-3 hour session every day. Although there were 2 breaks of about 1-2 months).
Before started pro-extender my length was 12.5 cm. And today it is 16.0 cm. My desired goal is to achieve length of 21 cm.
But still I don’t know how to use pro-extender effectively. I mean how to adjust the weight (900g, 1200g, 1500g). I tried but not able to adjust. Also there is no such user manual or video on internet which provides practical demo on weight adjustment.

Please let me know how to adjust weight and the effective way to use pro-extender.