By default running sudo apt update will check with the US server. To change to a mirror nearer, edit /etc/apt/sources.list
sudo vim /etc/apt/sources.list
To replace all http://us.archive.ubuntu.com/ubuntu/ to http://sg.archive.ubuntu.com/ubuntu/ run the global replace command within vim.
:%s/\/us\./\/sg\./g
Save and exit
:wq