How to install a second theme I purchased.

Hello, if anyone can help. This should be simple for someone. I have purchased a second theme but I don’t know how to set it up. My Hosting plan says I have unlimited websites so I shouldn’t Have a problem hosting another theme. But I currently don’t know how to set up the second theme I purchased. I currently log into my word press with my current auction theme and uploaded the second theme but Im scared to activate. If I activate my real estate theme will this overwrite my auction theme or do I need to some how activate a separate word press account to host my new real estate theme?