This command should do the trick
sudo sed -i 's/http:\/\/us./http:\/\//g' /etc/apt/sources.list
It will remove the 'us.' prefix in each of the addresses to convert them to addresses of the main server.
Of course replace 'us' by any other mirror you are using.
In depth explanation of command:
sed - stream editor for filtering and transforming text.
The -i argument is to edit a file in place.
Then 's/regexp/replacement/g', s specifying the search and
The g at the end being the argument to "globally" search.
The forward slash is the delimiter between the command and arguments except where it is escaped with a backslash.
Conclusion: replaces all occurrences of http://us. with http:// in the file /etc/apt/sources.list.
Note: if your replacement text has forward slashes, it may be clearer to use a pipe as the sed delimiter (ref http://backreference.org/2010/02/20/using-different-delimiters-in-sed/), e.g.:
sudo sed -i 's|http://us.|http://|g' /etc/apt/sources.list