CASE Statement to return Original Value

Hi, 

I have a variable 'County' which has a lot of blank data. What I am doing is creating a CASE statement to fill in the blank variables and to UPDATE the existing 'County' variable/

example:

case when 'Town' = 'Bristol' then 'South West'

case when 'Town' = 'Crewe' then 'Cheshire'

ETC

What I want my CASE statement to do is update the blanks based off my logic but return theoriginal values already held within the 'County' variable as the populated output in this variable, are correct

example: case when 'Town' = 'Bristol' then 'South West'

case when 'Town' = 'Crewe' then 'Cheshire'

then 'County' variable

Anyone able to confirm how to do this please?