You can top your balance up in the 'Billing' tab of your company panel. Go to 'Plan and balance' section and press on 'Top up" button.
Money can be transferred in 3 different methods using PayPal, credit/debit card and Bank transfer.
For Bank transfer verification 3 bank days are needed. PayPal (PayPal credit cards payments guide is here) and Credit card transactions are processed immediately.
Each billing cycle consists of 30 days. If the balance of a company becomes negative, you have 7 days to cover the debt. In case the debt wouldn't be covered during this period, the system will suspend the company despite your billing cycle. Once you top up the balance, the time your company account has been suspended will be added to your current billing cycle.
On the screenshot below you can observe a situation when despite a far next billing date, the account will be suspended in 6 days 13 hours.
Please, to avoid troubles, cover your negative balance in advance. You can enable auto-top-up option as well. For example to top up your account with 100$ once your balance gets -50$.