I personally use paid software to do this (Kofax Power PDF), but if this is something you do infrequently, you can try an online PDF editor (just google it) or you can also use Preview to redact text or add text (you can cover up the original text by inserting a shape. But that could get cumbersome if you have to do a lot. Hopefully you'll get some people who use free PDF editors that allow you to edit text in place that will chime in, but I don't use free software for that.
EDIT: Have you checked the App Store? Here's a free one, but I'm not sure if all the features they show will work or if some will prompt you to upgrade to use:
Download PDF Reader Pro: Convert & Edit by PDF Technologies, Inc. on the App Store. See screenshots, ratings and reviews, user tips, and more games like PDF…
apps.apple.com