SQL - substring query assistance

Hi experts,

I am trying to write a query that replaces the very first instance of the word 'Cath' in a string, with the word 'Echo'.

So far I have:

  update  card_proceduresextpop_
set reportdisplay =  REPLACE(SUBSTRING(reportdisplay, 1, CHARINDEX(reportdisplay,'Echo',1)), 'Echo', 'Cath')
         , SUBSTRING(reportdisplay, CHARINDEX(reportdisplay,'Echo',1)+1, LEN(reportdisplay))
  FROM card_proceduresextpop_
  where procedureabbr = 'Echo'
and substring(reportdisplay, 1, 4) = 'Echo'

but it gives me an error:

Msg 102, Level 15, State 1, Line 5
Incorrect syntax near ','.


Any ideas?

Thanks