To increase boot menu delay, first get a list of vms by typing the following in a terminal.
VBoxManage list vms
You can change the delay by typing:
VBoxManage modifyvm "You virtual machine name from above command" --bioslogodisplaytime 10000
Now start/restart the Ubuntu machine and hold down the left SHIFT key from keyboard unless you get the boot menu. Select ‘root …’ from menu.
We need need to remount the disk in read-write mode:
mount -rw -o remount /
Change the password of the required user, lets say user1:
passwd user1 ...