If you want to update just one package in Arch Linux, do the following
(please note that # indicates that the command should be run as root)
1) Search the package's name with
$ pacman -Ss keyword
(it prints out here the package's name and other stuff)
2) Run the following command to get the package's URI
# pacman -Syup | grep package's_name
(write the package's name you got in the previous command, now
you should get the package's uri)
3) Finally, copy the uri and run the following command to update it
# pacman -U package's_uri
Hope this helps somebody. As a side note, this is not the recommended
way to update a package. If possible, upgrade the entire system. However,
this is a good way when you just need to update one package and don't have
the bandwith to run a full system upgrade (for example, I recently needed
to update just the flashplugin package).