The best way to do this is to put together a small child theme of the theme that you would like to use, then overwrite the indexJournal.tpl template with your own. You can learn more in the PKP Theming Guide, but here’s a quick rundown.
Add this child theme to your plugins directory at plugins/themes/<your-child-theme>.
Enable the theme from the Settings > Website > Plugins page. Then activate it at Settings > Website > Appearance. Check that your site is still running. If you’ve gotten this far and the site is loading, then you’re running on your own theme.
Open plugins/themes/<your-child-theme>/templates/frontend/pages.indexJournal.tpl in a text editor and edit to your heart’s content. You can print the journal summary to the page with {$currentJournal->getData('about')}.
Glad you were able to get it working. You should know, though, that your changes will be overwritten the next time that you update OJS. So whenever you update, you will have to make the change again.
By separating the changes out into a child theme, you can protect them from being overwritten in a future update.