On the Settings->Billing page, you will see your currently selected plan. You can change the plan to add or remove more users to your account. Additionally, you can change the billing cycle (monthly vs yearly) to suit your payment needs.
If you change your plan to add users, Stripe will immediately charge your credit card for the prorated amount. Conversely, if you downgrade your plan, then your account will be credited the difference between the plans.
If you switch from monthly to yearly billing, Stripe will immediately charge your credit card for the yearly price, minus a credit for the unused part of the current month. Your next payment is then due in one year. If you switch from yearly to monthly billing, the unused part of your year is credited to your account and pays for your next monthly invoices.
To give you a concrete example of what this might look like, let's say you signed up on April 1 for the Basic plan at $29 a month. Half way through April, on April 16, you decide to switch to the Plus plan for $49 a month. You'll be charged $29 on April 1, the day you signed up. On April 16 you'll be charged $10: $24.50 for the 1/2 month on the Plus plan, minus $14.50 for the unused 1/2 month of the Basic plan. On May 1 you'll be charged $49 for May 1 to June 1.