If I pay annually how can I upgrade for a single month?

At this time, when you upgrade an annual subscription, you can only upgrade for the remainder of term you have pre-paid for.

If you only need credits for one month your options are as follows:

• If you have overages enabled on your account, you can send over your normal monthly limit and you will be billed on your rollover date for this charge.
• You can purchase Pay-as-you-go credit to supplement your subscription.