Just use the fabulous command REPLACE provided by mysql. The syntax goes like this:

update database_table set table_column = replace (table_column, "oldstring", "newstring");

For instance, to replace all occurrences of “modules” inside the linuxformat.gr drupal db with “sites/all/modules” I typed in:

update linuxformat_system set filename = replace (filename, "modules", "sites/all/modules");