Since this is the word you want to replace, it is not the content of the XML tag, but only part of that content - you might just want to check for text-based replacement options.
This will work if you have something like this:
declare @xml as xml
set @xml = '<root><info>well hello this is a <rating>very good</rating> example</info></root>'
You can then turn to XML and replace the content <rating>....</rating>with something else:
declare @replacement as varchar(50)
set @replacement = 'excellent'
set
@xml.modify('replace value of (/root/info/rating/text())[1]
with sql:variable("@replacement")')
select @xml
, , , <info> :
DECLARE @xml as XML
SET @xml = '<root><info>well hello this is a very good example</info></root>'
DECLARE @newcontent VARCHAR(1000)
SELECT @newcontent = @xml.value('(/root/info/text())[1]', 'VARCHAR(1000)')
SELECT @newcontent = REPLACE(@newcontent, 'very', 'excellent')
SET
@xml.modify('replace value of (/root/info/text())[1]
with sql:variable("@newcontent")')
SELECT @xml