Below tutorial could help how to add a new DNS record, A test with the content field value of your desired IP address - pointed to it. Make sure it’s proxied.
If just you need to change the IP address of the sub-domain, next to that DNS record is the button “Edit” which you click and change the IP address to the new one. Upon successfull change, you click “Save” to apply the changes.
More information about how to manage DNS records at Cloudflare dashboard for your domain can be read on the below article: