replace a value with sql

Hi Friends,
I have a table with many rows and I want to replace the values with sql query.
In my sql table named Table1 have two fieds ie UserId and UserName  
The rows are like this.
UserId                  UserName
1.                         gurpreet.singh.123
2.                         gurpreet.singh.433
3.                         gurpreet.singh.544
4.                         Sanjeev.singh.123
I want to replace replace only gurpreet.singh% to gurpreet.dhli%
How can I?
Please help.