Two names, and only one of them is cosmetic.
Label
What the portal calls the server. Change it on the server's Settings tab. Letters, digits, dots and hyphens, starting with a letter or digit.
Hostname
What the guest OS calls itself, set from the image's cloud-init at first boot. Changing the label does not change it. To change the hostname on a running server, do it in the guest:
sudo hostnamectl set-hostname web-01.example.com
Then make sure it resolves locally, or sudo gets slow:
sudo sed -i 's/^127.0.1.1.*/127.0.1.1 web-01.example.com web-01/' /etc/hosts
Reverse DNS is a third, separate thing
What the rest of the internet resolves your IP address to is the PTR record, set under Networking → Reverse DNS. See Reverse DNS.