You take it off with a tool. You didn't specify which collar, as there are 5 of them, 1 on the top tube, 2 on the bottom tube, and two of them on the sc top.
The SC top has the big outer nut collar, and under that is a smaller collar. The smaller collars are left hand thread.
They are designed to be removed with a spanner wrench.
See the link that was posted above.