You can access the theme files in the WordPress editor (Appearance -> Editor) or by using a program like Notepad ++ to open and edit them on your computer and then using FTP or your hosting cpanel to upload the changes.
If you are planning to make edits to your theme files (especially php) we strongly suggest you learn how to do so using FTP or via your hosting cpanel, rather than making changes in the WordPress editor. This will ensure that if you commit a coding error you’ll be able to easily and quickly restore your website from the backup you should have of any files you plan to edit. If you make a critical error (easy to do in your functions.php file) then you will get what’s known as the white scree of death and you’ll be locked out of your site so it’s very important to keep a backup of files you plan to edit.
To learn about FTP you can Google how to use your chosen FTP client (FireFTP and Filezilla are common ones, but there are others as well) and you will definitely find all sorts of tutorials to guide you.
No matter which method you choose, always make sure you have a backup of your files to fall back on in the event of a coding error.