You don't.
You would change the prices in your original application that you used to ceate the catalog, then export the file again to PDF and update it on the web site. You can edit text in a PDF file, but it gets cumbersome pretty quickly.
That's why it's a good idea to have the price list on a separate page (and no price information on the other catalog pages). This way, you can just recreate the price list, replace the page(s) for the price list and be done with it. That's a task of five minutes vs. days when you have to do it price by price with Acrobat.
A different approach would be to the price data in the file, stored e.g. as a CSV file. Your price items would then be form fields that would be filled dynamically from the hidden price list. When you want to update your prices, just update the embedded file and everything will update automatcially (as long as your clients use a PDF viewer that is able to handle JavaScript in PDF files). You probably cannot predict what your users are going to use, so you could take the approach of having a file with a dynamic price list, and then you flatten the document before you upload it to your web server. This way, the document is no longer dynamic, and any PDF viewer will display the prices associated with yoru products.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com