If you want to completely reskin your site, you'll need an appropriate skin package. Do you have one? If so, you should be able to install it and specify it for your site. I'd recommend creating a hidden page on your site, and verify that the skin works on your site for that page before you apply it to the entire site.
If you need to do more extensive testing or if you need to develop a skin for your site, then I would recommend that you make a copy of your site on a local computer for testing. When you are happy with what you have, you can still just install the new skin on your live site using the skin installation procedure.
You can become well educated about skins by studying the tutorials available
here. Look at the Skinning Tutorials especially. There are other administrative and DotNetNuke tutorials that you will also find quite useful if you need to create a local copy of your website and/or move information between the sites.
And, feel free to continue to ask questions. That is what this forum is for!