Upgrading attendee number

Hi - can someone help me pls. Im not great at this and my plan doesnt allow phone support.

We have an annual subscripton allowing 100 attendees.  It's the pro plan.

I need to add meeting capacity to 300 attendees as a once off.

Before I used to have a monthly subscription and was able to add this increased attendee as a once off, monthly plan. But now because I have an annual subscription, i cant work out how to do it. It doesnt allow me to upgrade for a month??

Can anyone help me please.