If you wish to cancel your Rectxt plan go to Settings, then you can find the 'Cancel' button at the bottom BILLINGS page.
Downgrade to Starter
A better option could be to 'Downgrade to Starter' - this way you you can still use the extension to import candidates and build talent pools for free (but you won't be able to text them). This could be a better option as you will still keep all of your historical Rectxt data (except for your phone number). If you want to text your candidates again then all you'll need to do is choose a new plan and follow the process.