I presume that what you really mean is the text at the top of the screen
in the bootloader? Like "Debian GNU/Linux 10 (buster)".
If that's what you mean, it's a part of the background image (splash),
automatically generated from an SVG, with the text hard coded into
live-build.
At the current time the only solution available to you, besides hacking
live-build, is to use a custom bootloader splash, which involves:
1. making a "bootloaders" directory within your config.
2. copying the folders for the bootloaders you're using from
/usr/share/live/build/bootloaders into it.
3. replacing the splash.
4. doing a build of course.