If the charge was missed, you have two options. By default, it will automatically be added to the next billing cycle. Otherwise, you can backdate and reissue the invoice. Below are the steps to backdate and reissue:
Go to the Invoices page and find the relevant invoice
Click on the […] and choose Send now
Choose what date you want to backdate the invoice
Click Send invoice now
Please be aware that if you've locked your ledger, you won't be able to create invoices in Xero. You will need to unlock the previous month first, and then run this process.