It is not possible to edit an existing invoice. However, you may make it void and create another one.
To provide customer specific discounts, you could add credits to a customer that will be adjusted against the subsequent invoices. So to correct any mistakes you can use the credits feature.
For customers who are looking to edit / void an invoice we support it manually. We have not built the feature to edit it via ChargeBee yet.
More details here: http://support.chargebee.com/categories/1622/forums/116888/topics/26003
Is it possible to edit or delete a pending invoice (for a subscription which does not have auto collection method enabled)? This is needed because sometimes we need to offer an extra discount to a customer for whom the invoice was already raised earlier, but has not been paid yet. This may also be required in case we added a charge by mistake to the customer, which later needs to be removed.