You can change price easily directly from the database.
You can use SQL Statement like this :
UPDATE <Table Name> SET price = CASE WHEN (price > 0) THEN (price + (Price * 5/100)) ELSE price END
(This was just an example, modify this according to your needs, see how manufacturers are connected with product and put that information in the condition area of UPDATE Statement and it should work)
Another thought was to export all the products and update them in excel then reimport.
I'm just unsure if this overwrites the existing product or just updates the product. I think the import does need some more features anyway as there does not seem to be a field for catagories, manufatuers and numerous other things, otherwise I would have done most of my origional product list in excel.
in order to update products from excel imports you have to make sure all your products have got SKU's defined
the best part of 1.5 version is when you want to update some information related to product just import the new excel file and it will update the information
BUT if you don't have SKU's defined for product new excel import will create duplicate products, so if you don't have SKU's defined it's better to delete products first and then import the new product excel file but yea then you have to manually do the category and manufacturer mapping.