Need help with an XML function

Oracle 19c

I am trying to write a function that takes an xml column, an xml path, and a new value for that item and replaces the current value with the new value and spits out the updated xml. This is what I have so far but it doesn't work mostly due to my limited understanding to xmlquery and xquery. What am I doing wrong?

CREATE OR REPLACE FUNCTION XMLFUN( XCOL IN XMLTYPE, XNODE IN VARCHAR, NEWVAL VARCHAR )

RETURN XMLTYPE

IS

BEGIN

SELECT XMLQUERY(

'copy $t := . modify(

if( $t'||XNODE||') then

(

replace value of node $t'||XNODE||' with "'||NEWVAL||'"