You should be able to change it from the update center. Try to follow this guide[1] or this one[2] if you don't have a graphical interface.
 
I don't have my Linux system now and I can't recall it from my memory, but that should be right.

[1] https://lifehacker.com/speed-up-ubuntu-downloads-and-updates-by-changing-your-5780680
[2] https://askubuntu.com/questions/104695/how-do-i-change-mirrors-in-ubuntu-server-from-regional-to-main