Imagine you have table called Information which could have two columns id and text_with_hrefs_with_or_without_target_blank, which we want to update
So for example one of many rows could have stored this value in the column text_with_href....
first href with target blank before
<a target="_blank" href="http://link1.com">Link1</a>
maybe another text and link with target
<a href="http://www.link2.com">Link2</a>
and last href link with target blank after href
<a href="http://www.link3.com" target="_blank">Link3</a>
I would like to update all similiar rows in table Information to add target=_blank to hrefs in those parts of rows which havent one
so the result for this row I mentioned would look like
first href with target blank before
<a target="_blank" href="http://link1.com">Link1</a>
maybe another text and link with target
<a href="http://www.link2.com" target="_blank">Link2</a>
and last href link with target blank after href
<a href="http://www.link3.com" target="_blank">Link3</a>
As pointed out in comments you better off doing it in your client/presentation code.
Now if you absolutely need to do this in the database, especially if it's just a one time thing, and assuming that you have one and only one link <a></a> per row you can do something as simple as this
UPDATE information
SET text_with_hrefs_with_or_without_target_blank =
replace(text_with_hrefs_with_or_without_target_blank, '<a ', '<a target="_blank" ')
WHERE text_with_hrefs_with_or_without_target_blank NOT LIKE '%target="_blank"%'