I've been able to reach the quick spring change system.
1. Remove the folding stock just like any folding stock on M4 series. Pull the pin and remove the folding stock.
2. After, twist anti-clockward the ring beneath the receiver and the buffer tube.
3. Pull the M4 Steel End plate. It will come with the buffer tube.
4. Remove the buffer tube.
5. Twist the pin anti clockward to remove it.
6. If you have an allen key, just use it. The biggest one you'll need. Twist the spring guide and be sure to hold it tight, cause there is a lot of tension on the spring. If you don't want to break a window, or kill someone who's in front of the hole... be sure to hold it. Or else it's gonna pop out very hardly of the mechbox.
If you don't have any allen key, just use the back side of the cap you've just unscrewed. Put it on the spring guide and twist. It works too.
That's it. Hope this is gonna help for those who wants to do a spring change on the STAR M4.