Pdf text editing is not working on Adobe Pro

I have purchased Adobe Acrobat Pro version because I want to edit some previous text in a pdf. I have been scratching my head since last night as the text never selects and give any option to replace it. Only I see options for "Add text" which sits on previous text, something I really don't want. Secondly, my application freezes and hangs incessantly. Please help, I need to edit pdf urgently.