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

SELECT

CASE

WHEN CHARINDEX(',',SampleString) > 0

THEN STUFF(SampleString,CHARINDEX(',',SampleString),1,';')

ELSE SampleString

END As NewString,*

FROM #tmp;

--updating the data?

UPDATE #tmp

SET SampleString =STUFF(SampleString,CHARINDEX(',',SampleString),1,';')

WHERE CHARINDEX(',',SampleString) >0