To purchase additional credits, go to Settings > Plan > Buy Credits. When you purchase extra credits, they will immediately be added to your account. If unused, these rollover credits will rollover from month to month until they are used.
If you find that you're buying credits often, consider enabling Auto Refill or upgrading your plan.
Note: only an account Owner or Admin has access to Billing and Plans. By default, the email address that initially created the account is the Owner.