a few ways to do this but the most often used is to make a string formula that you use inplace of the text field...
in the Field Explorer right click and select New
name it (e.g. Credit_vs_Invoice label)
add your formula here
if {HDRXXXXDEF.TITLE} = "RMA ACKNOWLEDGEMENT"
Then "This is NOT a CREDIT"
Else "This is NOT an INVOICE"
drag the formula field onto your report where you had the text box