ubun2

This is really easy. Please follow these steps
 

• Click on System->Administration->Synaptic Package Manager
• In quick search textbox type linux-generic
• Click and select all the old kernel versions you want to get removed (except the latest kernel)
• Right click and select the option Mark for Complete Removal
• Now click on the Apply button on the top toolbar
• Ubuntu will completely remove these kernels and the boot menu will be updated automatically with the latest kernel