Replacing current text with a fillable text box

Adding to the Document Geek's comment, you need to create fields for any form IN the form tool, not in the Edit Tool. Also since you are new to this, you can make a field and duplicate it but you must also change the name for any duplicated fields. So, for example, if you create a text field for the first name and duplicate that (for the last name). the user will fill out their first name and then that first name will show up in the duplicated in the last name field. So on the right hand side be sure to identify the first field as "first name" and the duplcated first name field as "last name."

That is probably the most common "oops" that folks learning how to make forms make. Fortunately it's an easy thing to solve.

The Edit tool in Acrobat is mostly there out of courtesy, to make very minor edits, and it often behaves unpredictably. Acrobat is not a word processor.

You'll need to go back to the origial application where the pDF was authored and make the edit there. Then add the form field in Acrobat.