DNSimple Dynamic DNS is a script to update the IPv4 and IPv6 addresses for a particular record in DNSimple using either the public IP addresses or interface IP addresses for a computer.
Edit the /etc/default/ddns file to fill in the parameters like below.
token='0123456789abcdefghijklmnopqrstuv'
account=1010
zone='example.com'
arecord=5
aaaarecord=6
ifip=false
The token parameter is your API token obtained from "Account" -> "API tokens" in the DNSimple interface. The zone parameter is the DNS zone, such as 'example.com'. The arecord and aaaarecord parameters are the DNSimple id of the record to update. The ifip parameter indicates whether the script should get the IP addresses from the primary internet interface or from an external service (icanhazip.com). You probably want ifip=false unless you know what you are doing.
$ ddns
I recommend copying the ddns script to your /usr/local/sbin folder and calling it from a script in one of your cron.* folders or from a systemd timer.