The short answer is that it depends on your package and which channels you're looking to change. If you have 'The Basics + Pik 5' or 'Pik Max' you can change out the 5 or 10 included specialty channels every 30 days at no charge.
The long answer:
There is no service charge to make changes to your combo/package, but if you add more channels, your monthly price may increase. You can make changes to your package by logging into your My TELUS account at www.telus.com/mytelus and then selecting 'Manage' beside Pik TV in the 'Home Services' box. From there select 'Add/remove channels' under 'Manage your Pik TV'. Once on this page, you can de-select channels you no longer want and select channels that you do want. You'll want to make sure that the number of channels you have before/after is the same otherwise your price may increase.