I recently needed to replace part of a URL found inside the body HTML of a page that was stored in a MySQL column. In order to do this I was able to use the handy
Replace() MySQL function.
UPDATE myPage SET pageBody = REPLACE(pageBody, '/oldirectory', '/newdirectory') WHERE INSTR(pageBody, '/olddirectory') > 0;
INSTR() function is not technically needed, but it prevents MySQL from doing an update on every row regardless of if the string is found.