This is one of those things I'm constantly forgetting, especially since I use Postgres for most of my work.
To replace the occurance of a string in a field with another, use the following. This replaces the value of the field with the value of the field where a text substitution has taken place:
MariaDB [DB]> UPDATE table -> SET field=REPLACE(field,'original','replacement');
The MariaDB site lists further details.