The titles are generated when you run update-grub, or grub-mkconfig.
IIRC, You can either disable os-prober (add "GRUB_DISABLE_OS_PROBER=true" to /etc/default/grub), to not automatically detect other operating systems, and then you configure them all manually in /etc/grub.d/40_custom, with whatever title you want. Then run update-grub afterwards to generate the new grub configuration with your titles.
The other option is to edit the file that is generated when you run update-grub, which is /boot/grub/grub.cfg. Look for the lines starting with "menuentry", enclosed in quotes on these lines are the titles as show in the grub menu. Note that if you change the titles here, whenever you do a update-grub (or when a new version of grub is installed) your changes will be overwritten.