This is just one more in a long line of "marking up PDFs" posts, which we have to update often because Adobe can't help themselves. They change the markup/comment interface with every new release, it seems.
The more comprehensive explainer is here; a shorter one for Acrobat X or XI is here. (That was a truly infuriating change, as the "Text Correction Tool" simply showed up and makes zero intuitive sense, but then again, Adobe gotta Adobe.) Anyway.
These days, most Acrobat tools - including comment and markup - are available in the Tools pane.
- Click Tools to open the Tools panel.
- Click Comment.
- This will open up the Comment and Markup toolbar.
- For simple text edits, click or make sure the "Selection tool for text and images" (good god, Adobe) is selected. This is perhaps better known by its common name "arrow."
- Then just click, select, and type as you would if you were editing the document itself. Deletions will be struck through, selected text replaced, inserted text will be marked with a caret.
And that's it! You're good to go.