declare @str varchar(50)=\'GoodLuck Markand\'
declare @replacedString varchar(50)
set @replacedString = replace(@str,\'Good\',\'Better\')
print @replacedString
Append empty space at start and end of the string then replace ' Good ' with ' Better '. Finally use Ltrim and Rtrim to remove the empty space at start and end of the string
DECLARE @str VARCHAR(50)='Good Luck Good MarkandGood Good'
DECLARE @replacedString VARCHAR(50)
SELECT rtrim(ltrim(Replace(replace(' '+@str+' ',' Good ',' Better '),
' Good ',' Better ')))