Is the info you want to update entered directly into a static HTML page? Or is it being pulled from a database and displayed on the page?
If you're just talking about updating simple things in an HTML page that are already there, like prices, company info, etc... that are easily found in the text, then you could just use a simple text editor like Notepad (and a little patience). Then FTP it into your domain directory, overwriting the old ones.
I'd recommend doing it on your computer and saving it there. That way, you can see what you've done first, plus you'll have a backup in case you need it later.
If this info is being pulled from a database, then that's different.
If your site uses a CMS of some sort, then there's probably already an easy way to log in and make changes. Even if you paid someone to make your site, it doesn't necessarily mean that they didn't build it on an existing product.
If you want to link to your site here, someone might be able to give more info, or tell if a certain script is being used that has an admin section.
If you don't want to link to it, look around the site. Are there any "Powered by " or "This site uses " type of links in the footer? How about the source code? Sometimes they're mentioned in there, or there will be copyright/license info comments.
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