WordPress has built-in editor that allows one to edit theme and its related files from the browser itself. In order to edit any installed theme one need to perform below mentioned steps:
1) Login into Administrator control panel.
2) Click on Design tab.
3) Click on theme editor.
4) Open theme file which one need to be edit.
5) Click on save.
To Open or view the file in theme editor click on file tab from the list. Open the themes file which one need to edit. One can edit HTML, CSS, PHP files of the theme.