First, decide whether you want to pay monthly or annually. Keep in mind you get a discount for choosing an annual billing option.
Once you select a higher plan and click the orange Change Plan button, you will be charged the prorated amount of the higher plan.
In other words, you will be charged the partial subscription amount based on the day you upgrade until your next bill date. You will be charged this prorated amount on the day you upgrade.
Similarly, if you decide to downgrade your plan, you will receive a prorated credit.
If you decide to cancel your plan, it will remain active until your renewal date and you will not receive any subsequent charges.