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You need a Content Management System (CMS) of some sort setup to do that. Usually the system generates the site and you control the content via a web based interface. This effectively separates the content from the design/mechanics of the site.
opensourceCMS and CMSmatrix are good places to check out these sorts of systems.
Everyone (at least everyone here) changes their existing websites. Many do it daily.
If you had an outside company put the site up for you, you will need to find the ftp username and password. You then log in to you FTP login (you can use Internet Explorer, or a program like filezilla) as suck the site to your local machine. Then you generally edit it there and republish whenever you are happy.
If you want to risk changing it right there on the server, without any regard for your web visitors looking at it as you make changes and risking oblivion (really not recommended if you are new to this) then you can use the Opera browser I think, to make changes on the fly).
Dixon.
