Installing process

I have started to install Arch Linux and I reached to step partitions. How I can delete the partition where my Debian Linux is located? I see only 465 GB of my disk. And use archinstall for graphical installing. And there is the point of boot loader. There I can choose GRUB as a boot loader instead systemd-boot. If I choose GRUB it will be more be easily to do shim 3.2.2? For example, I can not to rename of my boot loader or not?