To change a line can in posts across many in wordpress, can manually be a “pain-in-the-ass”-task. But can actually be done quite easily with a single line of mysql-code, executed from the db-admin-interface (phpmyadmin etc.)
UPDATE wp_posts SET post_content=REPLACE(post_content, 'text-to-be-replaced', 'text-to-insert')
To be more understandable, you use it like this
UPDATE Table-Name SET Field-Name=REPLACE(Field-Name, 'Text-To-Be-Replaced', 'Text-To-Insert');
With this one line, mysql searches the table ‘wp_posts' and replace all occurrences of the text-snippet it can find, and you are done!
The Other method is to use the wordpress-plugin Search and Replace, which runs from the wordpress admin-section (see Tools-menu!), and you can select which tables, in the database wordpress is installed in, to be searched!
In the mysql-command, the table is stated second after ‘UPDATE'!
See more: wordpress-support-topic and wordpress-plugin-search-and-replace.