Issue

Client want to change his monthly billing term to semi-anually (every 6 months) in WHMCS billing system. How to solve?

Answer

The client would need to contact an administrator to have their billing cycle updated. Once you have received a request from the client to update their billing cycle you would need to navigate to the product in the clients account and use the Upgrade/Downgrade button located at the top right. This will allow you to change the billing cycle option and create a new invoice if there is any charge for this.
But what if i deleted old invoice and i want to change billing cycle manually?

Question

My client wanted to change his billing term from monthly to semi-anually so i changed billing cycle in his product details, but invoice for one month was already generated. So i deleted this invoice hoping i will click generate due invoices link and it will generate new semi annually invoice for him. But this did not happend. Please how can be new propper semi-annually invoice generated quickly?

How client can renew his service from client area when there is no invoice, is that possible?
Answer

Hi,
Thanks for contacting WHMCS support. WHMCS will never generate a duplicate invoice for the same server on the same due date, even if the original is deleted. So in this case you'd need to move the Next Due Date forward/back by 1 day to get invoicing going again.
When you changed the client's billing cycle, ensure you ticked the "Auto-recalculate on Save" checkbox so they Recurring Amount value was adjusted accordingly too.
RESUME: In short how to change billing term and generate new invoice

After moving Next due date backwards for the service, im able to generate due invoices from client summary page or from whmcs main page (there is a link). But this will generate invocie for old billing term (monthly) so before i need to change Billing cycle in my clients service details (same page where i changed service due date). Tick recalculate on Save box so new price is adjusted.