The replace function in MySQL (for me) is:
update `mdl_glossary_entries` set `definition`= replace(`definition`, "thisisalessthantag", "<") where glossaryid=3;
The glossaryid being the value given to the glossary you are altering - this is NOT the id value given in the links to the glossary, I haven't tracked down how that relates to the table but found it simple enough to import my text and find the glossaryid value by browsing the mdl_glossary_entries table for the new entries.
A corresponding replace for the > is also required.