How to boot Hikey 970 with preinstalled Debian Linux?

Board based on the Kirin 970 - HI3670 Application Processor
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I am new to the Hikey platform. I have recently purchased a Hikey 970 board for a project and would like to use it as a Linux computer. The product description mentions that the board comes with preinstalled version of Debian Linux and AOSP. However, every time it boots with AOSP and I don’t seem to find instruction to switch the boot to Linux. Any help or directions on this are very much appreciated. Thanks!

You need to install the OS you want:
Third party images:
How to install: