You can manage cancellation settings only for your own dedicated servers. Address your upstream provider if you receive requests for server cancellation from clients with his servers.
Automatic cancellation of a dedicated server can be set both by you and by the client who has been using it. While the client can schedule cancellation only for the end of the billing period, you are empowered to perform the following:
Go Dedicated Servers -> Manage Servers and click Edit for the server you'd like to cancel and then click in the Status field. Set cancellation date:
If the server is due to be automatically canceled within the billing period, refunds will be issued according to your refund formulars.
You can update both your own and your user's cancellation settings. To discard cancellation, click and confirm your intention. To reschdedule, click and choose the new date.
To enable sending e-mail notifications about automatic server cancellation, go Dedicated Servers > Cancellation Settings. On the Dedicated Server Cancellation Settings page check the smaller box and in the bigger box, type the number of days in advance to warn your customers:
Read how to edit e-mail notifications.