open suse 13.2 - Access boot menu

I have OpenSuse 13.2 installed on my machine.
How do I access boot menu?

You mean grub2?
That should be shown automatically on boot.

That said, with btrfs (or LVM) as root filesystem (or /boot rather) it can happen under certain circumstances that a boot entry is set and cannot be reset, thus disabling the boot menu.
Remove the file /boot/grub2/grubenv to rectify that.