Well, of course! Under Membership in the menu, you can easily change your plan by selecting Change membership.

A plan upgrade takes effect immediately. When upgrading to Plus 1, you will pay a difference that is calculated from days already paid until your next billing date. You always see that amount when upgrading.

A plan downgrade from Plus 1 to Standard takes effect on your next billing date. You can continue to use the features of the Plus 1 plan until your next billing date.