How to choose kernel to boot

[mjd]

Hi all,

I have Rock64 2GB with Debian minimal Stretch on HDD, using the USB adapter from Pine64 shop.
System i even upgraded to buster/testing using just official Debian repo.
So far works excelent and I'm happy, except for the ancient kernel 4.14.

So here is my questions.
I plan to upgrade kernel, build it myself or wait for 4.18 from Ayufan.
But what if the new kernel will not boot, how can I recover?
Is it just possible to connect HDD to PC and change some conf file?

Another question is about HDMI.
Is it normal that I do not have output on TV (two tested) in early boot? Serial Console is needed to see SPI boot menu and his messages?
HDMI output starts workingvery late in 2-3 seconds before login.

Thanks for help for another ARM newbie?