How to use RAND() in SQL

I want to randomly choose a row in a column and change the value. I’m not too sure where Rand function goes.

To be more specific, I have a table with names in it but I want to randomly select one of the names from the first name column and change it to a different name of my choosing using the UPDATE and RAND() function. What parameters do I need inside the parenthesis as well?

I am using SQL Server Express

I tried starting the code out with

UPDATE tablename 
SET columnname = ‘new name’
WHERE RAND();