how to cancel a reserved ec2 instance

Reserved Instances cannot be cancelled, since it's a subscription for a period of time that will give you significant discount compared to on-demand pricing if you are running EC2 instances with attributes that match your RI.

However, if you made a mistake and never utilized your RI discount, I suggest opening a billing support case from the account where RI was purchased. In your request to support specify the AWS region, RI type and RI ID. This is not guaranteed, since you cannot cancel an RI, but I suggest asking support team if they can help with the solution if this was indeed a mistake and your RI discount was never used / applied to running instance.

Depending on the attributes of your RI, support team may suggest how you can modify the RI or potentially exchange it in case you have convertible RI. By modifying/converting it you could change it to match the attributes of some other running instance in a region, so that way you'd be able to continue benefiting from the discount.

Thank you for the response Nataliya G

I've put in a Support request asking for an RI cancelling. I hadn't meant to actually make an RDS Reserved Instance, I had just been looking.

I went to investigate what the pricing was for RDS Reserved Instances and the UI for the page isn't very user friendly and is very harsh. The process for purchasing a Reserved Instance for EC2, or purchasing a cost savings plan generally prefills with the correct suggestions and there's a "Search" button, you the "Add to Cart". It's very intentional and you know what you are signing up to.

However for the RDS Reservation process it's a Submit button, which I don't even remember Clicking (Does pressing enter on the keyboard submit? I don't want to try). The other BIG issue is that I CAN'T CANCEL. There's no major warnings indicating that you can't cancel a reservation. So whilst I was just trying to look at the price, I accidentally submitted the entry before being ready.

It would be good to see this UI/UX updated to be more in-line with the EC2 and Savings Plan process if that's possible. Otherwise at least add a WARNING message to the page indicating that it's not something you can cancel. Make the Submit button maybe have a different label like "Reserve Instance for $851.40" or something more useful.

Ideally you'd also have a 24hr window whereby we can cancel Reserved Instances. I don't want my cat or kid playing with my keyboard at the wrong time causing me a MASSIVE bill.