By setting init=/bin/sh in the editable grub command line. You can google how to get there exactly. This directly starts a root shell instead of the normal init process. (it is also in the raspberrytips link that were already linked in another comment).

To note: This also means that anybody who has physical access to your console during bootup can root the system. If you do not want that, you need to set a harddisk encryption password (and optionally also a BIOS password, but the latter won´t protect e.g. against disassembling the computer and putting the HDD into another Linux box).