18.04 - How to switch to a signed kernel in Ubuntu 18

I have multiple kernels installed in my pc. In which an unsigned version is active now. I want to switch back to an older signed version of the kernel.

When I run the following command: dpkg --list | grep linux-image

This is what its listing:

1. ic linux-image-4.15.0-29-generic 4.15.0-29.31 Signed kernel image
generic


2. rc linux-image-4.15.0-39-generic 4.15.0-39.42 Signed kernel
image generic


3. ii linux-image-4.15.0-42-generic 4.15.0-42.45 Signed
kernel image generic


4. ii linux-image-4.15.0-43-generic 4.15.0-43.46
Signed kernel image generic


5. ii linux-image-generic 4.15.0.43.45
Generic Linux kernel image



6. ii
linux-image-unsigned-4.19.5-041905-generic
4.19.5-041905.201812031110 Linux kernel image for version 4.19.5 on 64 bit x86 SMP

The 6th kernel is active now. Which is not signed. And due to that facing lots of issues in updating and installing different packages.

I would like to switch to 4th kernel (Linux-image-4.15.0-43-generic 4.15.0-43.46) signed version.

How to do that? Also is it possible to remove all other kernels?

Hold down (right) SHIFT to display the menu during boot. Use the 4th kernel by selecting it in "Advanced options for Ubuntu" submenu in GRUB during startup. Then proceed with these instructions to remove the extra kernels.