This is an advanced guide and requires understanding of web development concepts. You’ll also need to setup a local development environment to edit your theme files. To do so, you can follow the instructions in the Ghost documentation.
To edit your Ghost theme files, you first need to unzip the theme archive. This will give you access to the necessary files. Open the unzipped theme folder in a code editor of your choice, such as Visual Studio Code. When you’ve finished making changes to the theme code, re-zip the theme directory for uploading back to Ghost. You can use the following command on your terminal: 
npm run zip
This will create a new zip file in the dist folder. You can then upload this file to your Ghost Admin panel. If you have more questions about editing your theme, you can check the Ghost documentation.