It is recommended to remove old kernel version to clean up the boot menu and to get some extra disk space.

1. Check which kernel version you're using:

uname -r

2. List all installed kernels and headers on your system:

dpkg -l | grep linux-image
dpkg -l | grep linux-headers

3. If you're not using the latest kernel version, try rebooting your system first.

4. You can remove old kernel versions automatically:

sudo apt-get autoremove

5.a. Or you can remove them manually one by one:

sudo apt-get purge linux-image-x.x.x.x-generic

Be sure, NOT to remove the version you're using! 

It is also recommended to leave the previous version on your system.

5.b. When you're done, update your Grub:

sudo update-grub2

6. Reboot your system.