From the menu select Custom Templates then click Edit next to the template you want to edit.
You can search within the web editor at any time by pressing Ctrl-F (or Cmd-F on Mac).
If your template is deployed from a Git repository, you will have the option to edit the Git settings if required. You can also click Reload custom template to update the template from the remote repository.
Make the required changes to the template then click Update the template.