Leland,
What theme are you using?
To modify template files, you will need to use FTP / SFTP to transfer the files. I would NOT recommend making more complex edits, such as you are suggesting, by using the wp admin. You’re likely going to white screen your site at least a few times while making changes (mistakes) and then you will have a much harder time removing the mistake if you are not using FTP.
Use a file.code editing program such as notepad++ (free) to open the file in to edit.
FileZilla is a free FTP program.
I would recommend you slow down a bit and take your time with learning, and then editing, so that you do not get in over your head and confused If you are aware of how to modify files & upload them to your site, then great— I am just sensing my some of your questions that perhaps you are a bit new to this.
If you could give me a rough idea of your level of wordpress experience I can know how I can best guide you to make some of these changes.