It’s on line 365 in your style.css file. Are you not using a program to edit your files? If not, I highly recommend that you do, then it will be easier to locate things like this when someone tells you what line a piece of code is on. I would use FTP and a CSS editing program. If you need to learn how to set that up you can search online for tutorials.
Other than that you would have to search your css file for the .menu li code. Control + F if you’re on PC, Command + F if you’re on Mac
.menu li {
border-top: 1px solid #000;
}