After upgrading your OS, you will find that there are multiple kernels on your system. This is because, new kernels have been installed, while your old kernels still remain. In order to free your GRUB and system from the old not-needed kernels, you can do the following.
1. Find out your recent and the latest installed kernel.
varsha@varsha-laptop:~$ uname -r
You will get a similar output. This is the latest kernel installed on your system, and the one in use. This one is to be kept. Do not remove this. The kernels to be removed must be older than this.
2. Go to your Synaptic Package Manager via System-> Administration.
3. Hit Ctrl + F, and type linux-image-2, or type it in the Search box, and click Base System on the left pane. You get the list of installed kernels.
4. Carefully remove the old ones, by clicking them and selecting Mark for Removal. Then click Apply. Be careful. Do not remove the current kernel.
At the next system startup, you will see that the old kernels have been removed.