To append a text after the content for a column:

UPDATE `articles` SET introtext= concat(introtext, ‘</div>’);

To add a text before the content for a column:

UPDATE `articles` SET introtext= concat(‘<hr class=”system-pagebreak” /><div style=”display:none;”>’,introtext);

To replace text in the content for a column:

UPDATE `articles`
SET introtext = REPLACE(introtext, ‘<hr class=”system-pagebreak” />’, ‘</div><hr class=”system-pagebreak” /><div style=”display:none;”>’);