how to specify mirror for installation

From: hl <hlyg2023_at_outlook.com>
Date: Sat, 27 May 2023 06:00:35 UTC
i use "pkg install" to install, default server is very slow, 10KB/S, it 
takes hours to install xorg or firefox

to make matters worse, sometimes it is stalled, i have to Ctrl+C and 
restart, partial downloading is buggy

how can i change default server to some mirror? Thanks!