XIAOMI Redmi 6: how to change or remove the pin of the sim? I have XIAOMI Redmi 6 phone, model M1804C3DG. Android version: 8.1.0, kernel version: 4.9.77+ .

There is no options in the menu to remove or change the pin of the sim. 
I have enabled developers options too, but no options are displayed related to the pin of the sim.
If I insert this sim in another phone (ios operating system), the options to remove or change the pin is displayed in the menu, and it works: this means the sim works properly.

How to remove the pin, using this XIAOMI Redmi 6 phone?

Thank you
Go to Settings -> Additional Settings -> Privacy.
Find SIM Lock section, select the SIM you want to change the pin, then click on "Change SIM PIN".