Unfortunately, you can't update your theme via WP Dashboard, you can do this via ftp.
Before you update your theme, kindly backup your files first if you have done some customizations (some files like custom.css, etc). After you are done with the backup, download the latest theme on themeforest.net, unzip the downloaded file and you should see abundance.zip. Kindly unzip this file and you should see two folders, _MACOSX and abundance. Now, connect to your remote server via ftp and go to public_html > wp-content > themes > abundance. Replace all the contents of the abundance folder on your remote server with the one you have downloaded. Hope this helps. :)