Actually it is simple to use the recovery mode.
I entered recovery mode and pressed alt-F2 to loging as root. Changing cmdline.txt from the GUI does not work.
After logging in as root with password raspberry, check if there is any /boot directory. Most probably it will not be present.
If not then create it using mkdir /boot
Then use vi editor to create cmdline.txt.
in vi editor change the root to mmcblk0p6.
My cmdline.txt contents looks like
dwc_otg.lpm_enable=0 console=ttyAM0,115200 console=tty1 root=/dev/mmcblk0p6 rootfstype=ext4 elevator=deadline rootwait
save the file.
then it is important to do sync.
Then boot the board. Cross check using GUI if cmdline is really changed.
My PI B+ is working now