Install the memory card into a computer.

Edit cmdline.txt. Append "init=/bin/sh" to the parameter string.

Save the file, then reinsert the mem card into the Pi and boot it.

Remount the file system as read/write: mount -o remount, rw /

Change the password: passwd pi

Bring the the Pi up to normal run level: sync; exec /sbin/init

Test password.

Shut down, remove the mem card. Use the other computer to edit cmdline.txt to remove the init cmd.