Look in the /boot directory to see if there is a kernel image of the version that you want. Then check your boot loader to see if it is loading that image or if it is still loading the image that you have been using prior to the upgrade.
If your boot loader is loading an image with a generic name such as vmlinuz then see if that is a pointer or a regular file. If it is a pointer see if it is pointing to the old kernel image. If so then delete it and create a new pointer that points to the new image. If it is a regular file then delete is an copy the new kernel image to that file name.
Be prepared to boot a live Linux CD to fix anything that you might break.
Doing these things by hand is SO 20th century. Too bad this wasn't/couldn't be done using some nice GUI system administration tool.