the STUFF command can help you with that one:
CREATE TABLE #tmp(ID int identity(1,1) not null primary key,[SampleString] varchar(150))
INSERT INTO #tmp(SampleString)
SELECT 'this is a test, hello World 4.2 test' UNION ALL
SELECT 'other data, but with, like two commas' UNION ALL
SELECT 'other data with no commas at all.' UNION ALL
SELECT 'another row, with a couple of commas, i think'
--selecting the data
WHEN CHARINDEX(',',SampleString) > 0
END As NewString,*
--updating the data?
SET SampleString =STUFF(SampleString,CHARINDEX(',',SampleString),1,';')
WHERE CHARINDEX(',',SampleString) >0
--help us help you! If you post a question, make sure you include a CREATE TABLE... statement and INSERT INTO... statement into that table to give the volunteers here representative data. with your description of the problem, we can provide a tested, verifiable solution to your question! asking the question the right way gets you a tested answer the fastest way possible!