REPLACE is a string function that allows to replace particular characters in a particular string.
CASE WHEN is like a conditional function that takes decisions base on the conditions provided in the statement.
In your case the below given code can be used because the condition in robust or static .
CASE
WHEN fieldname = 'SQL Tutorial'
THEN 'Code Project Tutorial'
ELSE fieldname
END AS 'fieldname'
But the other solution can be used if you are checking if the fieldname contains "SQL" sub string.
For example as follows:
CASE
WHEN CHARINDEX('SQL',fieldname)>0
THEN REPLACE(fieldname, 'SQL', 'Code Project')
ELSE fieldname
END AS 'fieldname'
In the above code the condition is dynamic and it is more feasible to use a replace function.