If you already added the payment information, changing products is pretty straightforward.
Click on Manage licenses on left sidebar:
Click on other plans:
Depending on the product you are currently on, you will see certain options.
If you are using The Critical Path, for instance, you will see the options to change plans and also to switch to Meeting Mastery:
Click on the Meeting Mastery button to change to that product.
Please note that the picture above is for illustration purposes. Features and prices might vary.
If you have not added the payment information yet, you will have to do that first in order to change plan (even if you are during the trial period).
To do so click on enter billing information.