A PTR record for a single IP address can be also delegated to your very own dns, so you could do it yourself. Usually this is done by ISPs.
It is also possible that your ISP has a web page to allow you to define it.
If neither case applies, then you will need to ask them to change the PTR record on your behalf. Your mileage may vary!
Anyway, if an IP address of yours is
184.108.40.206 and you have a DNS record A pointing to it like
www.mydomain.tld, and if your DNS server has been delegated for the reverse lookup, the a minimal PTR record would look like this:
220.127.116.11.in-addr.arpa. IN PTR www.mydomain.tld.
Be very careful with the trailing dots: the DNS won't forgive you!
We never use trailing dots, the DNS cannot leave without: it's part of the delegation tree.