Arsenal can be downloaded from the Arsenal website:
The downloaded file will install the Arsenal Launcher. The launcher will download all needed game assets from the server. With the launcher, you will always be up-to-date with the latest version of Arsenal. Every time the launcher is started there will be a check for a new patch or update. New patches or updates will come with a changelog.
Mandatory updates will also be pointed out in the game itself. When a mandatory update is pushed, the game cannot be played until the update is done.