Everything is in Billing: Overview, Plans, Invoices and Audit Logs.

ChangeWhen it applies
UpgradeStraight away, charged pro rata
DowngradeAt the end of your billing period
Add usersStraight away, charged pro rata
Remove usersAt your next renewal
Annual to monthly, or the other wayAt your next renewal

Plans lets you choose monthly or annual billing (annual saves up to 30%). Prices show in INR or USD depending on where you're accessing Hyring Insight from, and the currency can't be changed. See Which currency do I pay in, and is tax added?.

Invoices lists every invoice as Paid or Failed. You can download any invoice, or Retry a failed payment.

The Billing overview
Your invoices