Only text fields are supported:

The following functions and statements can be used with ntext, text, or image data.
Function statement
Datalength readtext
Patindex set textsize
Substring updatetext
Textptr writetext
Textvalid

Topic: Text Field
1: replace

-- Create a data test environment
Create Table # Tb (AA text)
Insert into # TB select 'abc123abc123, ASD'

-- Define the replaced string
Declare @ s_str varchar (8000), @ d_str varchar (8000)
Select @ s_str = '20160901' -- string to be replaced
, @ D_str = '000' -- string to be replaced

-- String replacement
Declare @ P varbinary (16), @ postion int, @ rplen int
Select @ P = textptr (AA), @ rplen = Len (@ s_str), @ postion = charindex (@ s_str, AA)-1 from # TB
While @ postion> 0
Begin
Updatetext # TB. aa @ P @ postion @ rplen @ d_str
Select @ postion = charindex (@ s_str, AA)-1 from # TB
End

-- Display Results
Select * from # TB

-- Delete the data test environment
Drop table # TB