Install

Euphony installation is initially created on USB stick, by writing Euphony image to it (from your desktop OS on another machine).
The ability to install Euphony on an internal drive (or any other drive), using Euphony machine itself, comes after you buy the license and fully register your USB Euphony installation. For that purpose, you will use 'Install' section of the Settings page. 

'Install' section will not appear for Trial registrations! This is the only major limitation of Trial registrations.

'Install' function uses an additional image of the OS file-system that is present only on USB image installation.
This image is not present on 'Install' target drive (on Euphony installation created by 'Install' function). That means you cannot use 'Install' function again on the Euphony installation created by the 'Install' function (i.e., if you use 'Install' to install Euphony on another removable drive you cannot boot from that drive on another machine and expect to use 'Install' again to install Euphony on its internal drive).

If you are fully registered and you are still missing the 'Install' function, you have to create a new Euphony USB stick, boot from it, register it using the current registration code and then use 'Install' which will then be present.

Install release version

As mentioned in How to update Euphony & Revert feature article Euphony has two types of updates: System and Feature updates. 

Since feature updates only change specific files on the installation drive and do not update the installation image file, Euphony installation created by 'Install' function will only ever contain the latest System update - even if you apply the latest Feature updates to the USB Euphony installation before performing 'Install'.
Consequently, there is no need to perform 'Complete installation' function (which gives you the ability to update your USB installation) and apply latest Feature update if you only ever use USB installation to perform 'Install' (or 'Repair') on internal drive. 

'Repair' function

It is useful to always keep the original Euphony USB drive at hand. You may decide to buy another license and Install Euphony on another machine.
It is also possible that your internal drive with Euphony installation will become corrupted - causing issues with normal system operation. In that case you can boot from Euphony USB drive and again use 'Install' function with your internal drive as a target. After clicking first 'Install' button, when the program detects that target drive already contains Euphony installation you will be presented with two options: 'Install' and 'Repair'

'Install' will recreate your drive from the beginning: partition table and partitions will be re-created, partitions will be re-formatted, OS file-system image will be unpacked and configuration re-initialized. As a result you will lose all data on your installation, including music, database and all the configuration.

'Repair' will use OS file-system image to re-create OS partitions but it will leave your data partition (where music, database and almost all your configuration resides) completely intact.

Please do not use 'Install' or 'Repair' on existing installation without consulting first with support - whatever problem you have, it is much better to exhaust other options - give us the chance to determine the exact cause of the problem. This may be a bug - a systemic problem which could possibly re-appear again after re-installation, or misconfiguration that can be avoided in the future.
You also cannot be sure which one you should use. If corruption is on the system partition 'Repair' will be enough. If corruption is on the data partition, it is still possible that we can connect remotely and fix it without resorting to complete overwrite of your installation. Even if you cannot boot or access Euphony Web interface due to problems on data partition, you can boot from USB, enable remote control and let us try to fix the problem on your internal drive data partition. 
Sometimes the corruption will be un-fixable and you will have to overwrite your installation but you can at least be sure to always have a way to get Euphony back. 

If you have used Expert settings in the past, before using 'Repair' make sure to get out of ramroot first and to un-bridge your Ethernet interfaces so you can avoid discrepancies between the configuration (which is on the data drive) and OS state (which was re-initialized during repair). Some expert options are just not robust enough to withstand such unsynchronization.