Always back up before applying a mass update. Show all records and place your cursor in the price field. Select Records > Replace Field Contents > Replace with calculated result and then Specify and type into the calculation box (where price is your price field) : price + price * .1 ... for 10% increase. Then click Replace. If this file is being served, protect from record locking issues.
UPDATE: However, I believe that it would be better to have a table with your prices and an EffectiveDate (and add new records when you change your price) or if you ALWAYS will mass update your prices, have another table with only the percentage of increase. Why? Because it will assuredly come in handy to have that kind of audit trail on your prices.
Sorry about the delay getting back to you. The solution worked. Thanks