Hello @MichaelG, Welcome to the /e/OS users forum.
e-4.0-a15 is not a simple system update, it is associated to an Android level upgrade.
Because of encryption method change, a “factory-reset” that erase all your apps and personnal data is expected during the upgrade process
/e/OS official star2lte download
[HOWTO] Upgrade my /e/OS to major Android version without loosing /data