How can I update this array?

Here's my array when I do a var_dump:

array (size=3)
  'eventdate' => string '2016-08-27' (length=10)
  'eventtype' => string 'transaction' (length=11)
  'runningbal' => string '32.50' (length=5)

The name of the array is "$event."

I need to update "eventtype." My plan was to do a "foreach" in order to isolate each of the variables and then change "transaction" to "payment."

When I do this:

foreach($event as $sport)
{
 echo $sport['eventtype'];
}

I get this: Warning: Illegal string offset 'eventtype' in C:\wamp\www\patient_focus\arrays.php on line 74

What am I doing wrong?