To change your mirror, open the file /etc/pacman.d/mirrorlist, uncomment a mirror near you, and move it to the top of the file.
rankmirrors will take a list of mirrors and arrange them in order of speed. If you want to find the 5 fastest mirrors where you are, pass it your mirrorlist with servers near you uncommented along with the "-n 5" option and redirect the output to a file, e.g.
cp mirrorlist mirrorlist.bak
rankmirrors -n 5 mirrorlist >mirrorlist
That will save a backup of your current mirrorlist to /etc/pacman.d/mirrorlist.bak and rank your mirrorlist file.
If you want to really increase pacman's speed, take a look at powerpill and reflector (the link is in my sig).
Reflector From ArchWiki
Reflector is a script which can be used to retrieve and filter the latest mirrorlist from the MirrorStatus page.
It can be obtained from the community repository:
# pacman -S reflector
First backup your mirrorlist:
# mv /etc/pacman.d/mirrorlist /etc/pacman.d/mirrorlist.backup
The following command will filter the first 5 mirrors, sort them by speed and overwrite the file /etc/pacman.d/mirrorlist
# reflector -f 5 -r -o /etc/pacman.d/mirrorlist
For all available commands run
# reflector --help