Hey!
Thanks for contacting us!
You can cancel at any point before the end of the trial to avoid being charged. Cancellation always takes effect on the next renewal date so you'll retain access to the paid tools and data, even if you cancel on day 1.
We have a guide with a video on our Help Hub which will show you how to cancel.
Just a quick reminder that once your free trial expires your Moz Pro Campaigns will be archived and no new data will be collected. However, if you want to come back you can restore your Campaigns. We keep all your historical data for around 12 update cycles. For weekly updates, that'd be 12 weeks and 12 months for monthly updates. A couple notable exceptions would be for rankings and link histories, which we'll keep forever.
I hope this helps! Please let me know if you have any further questions about this. 
Eli