Yes. Simply go to “Account settings” from inside the application. The first tab is “Plan”. You will see your current plan listed there right at the top. To change your plan select the plan you want to switch to from the drop-down list at the bottom of that page.
If you UPGRADE, your credit card will be charged for the higher amount on a prorated basis. For example, if you purchased a $49/month plan and you want to upgrade to the $99/month plan, you will be charged the prorated difference for the month you are in for that month, and then your monthly payments will increase to $99/month for the next billing cycle. The same applies for annual plans.
If you DOWNGRADE you will not receive a refund on past purchases however your monthly or annual charges will be downgraded for FUTURE billing. For example, if you are on a $99/month plan and you want to downgrade to a $49/month plan, you will receive no discount for the remainder of the month you are in but your future payments will be reduced to $49/month at the next billing cycle. The same applies for annual plans. For this reason, particularly on annual plans, it is wise to only downgrade right at the end of your billing cycle.