If you just want to change the size of the text and do some style changes to “Leave a comment”, you can add the following to the Stylesheet (style.css) file found in Appearance > Editor.
.comments-link a {
Put your CSS here
}
For example, this will change the font to size 18px and turn the color red:
.comments-link a {
font-size: 18px;
color: #88000;
}
It is best to create a child theme if you are going to start modifying a theme, this way if update the theme you do not erase your theme changes. Child Theme info – https://codex.www.ads-software.com/Child_Themes
To move the “Leave a comment” to a different location requires changing the code of the theme a little. If you post a request on where you want it, I am sure someone will post it up to help you out.