if you use a minecraft dedicated hosting website, ask them to update it for you through tech support. if they can't, it might be because the plugin did not update. if you don't have a dedicated host w/ tech support, delete it from your "plugins" folder and re-install the new version.

I mean like if the plugin is only 1.6.4 how can I change the plugin to 1.7.2 because sadly the plugin can't be on my server because it uses 1.6.4

place the new file in your plugins folder as you did the 1st time, if its the same name over right it

Yes, but if you do have to update it, that how you do it lol

All you have to do is redownload it from the bukkit website