Write a SELECT statement joining the two tables which outputs the PK of the table you want to update, the existing domain value, and the value you want to change it to.
Check that the output matches your requirements.
It's then a simple matter to convert this SELECT into an UPDATE...FROM. If you are unsure then post the SELECT and someone will step in to help you.
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden