Once nothing is bound to fedora-23, then you can remove it with "sudo dnf remove qubes-template-fedora-23" in your dom0 terminal. (Don't run it yet). Be sure fedora-26 works as intended before you remove the old template, i.e. test if internet works in sys-net and sys-firewall with the fedora-26 template.
Run "qubes-global-settings" in dom0, switch any possible fedora-23 to fedora-24. Test if the changes work, in particular the dom0 update (just to be on the safe side).
Then run "qvm-ls" in dom0, and detect any possible AppVM running fedora-23. Change it with either the GUI VM-settings for each respective AppVM, or use the qvm-prefs to do it. Both net same results, whichever you prefer GUI/Terminal.
A bit redundant, but you may also check if the fedora-26 can start on other critical VM's, if copy/transfer from/to fedora-26 VM's work, whether their internet works, and so on.
Once everything is untied and conirmed working, then you should be able to use the "dnf remove" command above mentioned initially.
Any primary templates pre-installed, or primary ones you got from the qubes repositories, should only be removed with "sudo dnf remove qubes-template-
'template-name'". Do never, EVER, use "qvm-remove" for these primary templates. If I'm not mistaken, this is still possible user-mistake to pull and the system won't prevent you from doing this mistake. Any templates you cannot with normal means re-name, has RPM update enabled, etc. are typically the ones that are primary and require the "sudo dnf remove".
Any template copies of the primary Qubes templates you can remove the same way you remove AppVM's, with "qvm-remove VM-name". Just be sure you don't use it on the primary ones like fedora-23, fedora-26, debian-8, debian-9, whonix-gw, and so on. While fedora-26-copy instead must be removed with "qvm-remove", never use qvm-remove on the primary ones.
Also be sure to keep a watch out for when fedora-26 has end-of-life (EoL). Check here https://fedoraproject.org/wiki/End_of_life
for the known ones. Fedora-26 is still unknown, but if you check here
Quote: "Fedora generally develops new releases over a six month period to provide a regular and predictable release schedule. The bi-annual targeted release dates are May Day (May 1st) and Halloween (October 31) making them easy to remember and for avoiding significant holiday breaks. Changes to this standard must be approved by the community-elected Fedora Engineering Steering Committee (FESCo)." https://fedoraproject.org/wiki/Fedora_Release_Life_Cycle
Basically, new release every 6 months, but it takes extra time to fully update the release to stable, including older versions having further support for a while afterwards. They are generally kept updated in rougly 400 days after first release. So be sure to check when to upgrade to fedora-27. Qubes will still release qubes-based-updates after fedora EoL, so you might get the illustion its still maintained, when it's not. Keep an aye out for these dates.