You should be able to add the pgdg repository to your system and then install through apt as normal. Scroll down to the "PostgreSQL APT repository" section on this page:
We want to run 9.3 on the above distro, which comes with 9.4 as standard (in the distribution). However, we note that there are only 9.5 packages in the postgresql 15.10 repository. Can somebody flip the switch to build these? We really aren't ready to upgrade to 9.4 at the present time.