I don’t know much about kernels thing, but I tried once to revert to an earlier kernel, and wow, did it hose my linux install- I had to do a timeshift revert to get back my system, which thankfully I was uncharacteristically “smart enough” to do a snapshot before trying out the kernel th8ng. I have since built a computer and have dedicated a 1 tb drive to just timeshift backups, and macrium reflect backups for my 2 windows installs on a seperwte drive, and I also duplicate the backups to anothe r 2 tb external ssd. With the new NVME drives, it takes only 2 1/2 minutes to do a timeshift backup of linux. Thr macrium reflect for windows takes a bit longer, but nowhere near as long as it used to with ssd, or with hdd especially. 8m ,owing the speed of the NVME’s
I honestly have never needed to change a kernel. The default have always worked for me. I just know you can. I have never used the kernel GUI changer in the update manager but maybe it is broken. Did you use that? I think that you can load kernels into the kernel folder and then choose from them at boot up from the grub menu?