If you want to remove old kernel versions from your ubuntu system easy way is using ubuntu tweak.
Install Ubunt tweak
Follow this guide to install ubuntu tweak
Once you have installed ubuntu tweak use the following procedure to remove old kernels
Step 1 -- Select "Package Cleaner" on the left and ""Clean Kernel" from the right panel.
Step 2 -- Press the "Unlock" button at the lower right, enter your password.
Step 3 -- Select from the displayed list the kernel images and headers you wish to remove. The kernel in use is not listed.
step 4 -- Press the "Cleanup" button at the lower right to remove the selected kernel images and headers.