How can one update the current OS Menu options that appear right after one enters the CMOS PWD?

I installed Debian 9 since installing Trisquel 8, so my current OS Menu - what appears after entering my CMOS PWD - has Debian's options at the top, then Trisquel's.
I tried to get Trisquel to update its default choice of Linux kernel, and it seemed to work, but when I rebooted, the OS Menu remained as described above.
RELATED ?s:
1. from within Trisquel can I get use a terminal CMD to get Trisquel to both:
a) overwrite the OS Menu created by Debian?_______
b) install by Default on future computer restarts?_____, IF NOT, then...
c) install by a new option on the Trisquel list of OS options?_______
d) IF YES, please include verbatim terminal CMDs to actually DO those jobs!_____
2. IF I can't do at least point 1.a), then Can someone tell me HOW to DO all of above, with the OS, etc that I'll need?

PS I've used GNU/Linux OSs since 2007, but I can't remember terminal cmds too well. Another related ? is:
3. What CMD(s) is/are necessary to increase the time delay between the OS Menu's 1st appearance, and its activation of its Default 1st choice?_______