Use of STUFF function in SQL Server?

Stuff function is used to replace the occurrence of the String in a Expression based on the start value and length value.

Syntax : 

STUFF ([CHARACTER EXPRESSION],[START],[LENGTH],[REPLACEMENT STRING]

Example :

SELECT STUFF('JK SQLBLOG',7,1,'SERVER ') 

In above example replacement string 'SEVER' will be Inserted in the character expression.

It search for the character position 7 in the character expression and after that it will insert the replacement string.

Output :

JK SQLSERVER LOG

Use of Replace function 

http://jksqlserver.blogspot.ae/2016/03/use-of-replace-function-in-sql-server.html

Replace Vs Stuff

http://jksqlserver.blogspot.ae/2016/04/difference-between-replace-function-and.html