The latest Apache version is installed from OS repository on Plesk server. Is it possible to upgrade it to any newer custom version from 3rd-party repository like IUS?
Plesk does not support custom Apache versions. Only default one that is provided by OS repository is supported.
For any available Apache updates in OS repository execute Plesk installer and apply all the updates.
Install all the available updates by running Plesk > Tools & Settings > System Updates(Onyx only) or Updates & Upgrades or manually by running Plesk installer or system update via SSH:
# apt-get update && apt-get upgrade (For Ubuntu/Debian)
# yum update (For CentOS/CloudLinux/RHEL)