If you click on Settings>Client Letter it will take you to the editor.  From there select from the Letter navigation box on the left of the screen States>Oregon>and then the appropriate form from the list clicking on the + box to expand the list each time.  Once you're in the editor select the component you want to change and you should be able to edit the text.

Another alternative is in the Forms view bring up the letter on screen and then right click on the letter.  This will take you to a screen where you can customize just that one letter.  Take note of the warning that comes up.  This is a static change and will not update if you make changes to the return, so only do this if you are finished with everything else on the return.