I just installed XFCE yesterday and when I finished updating I had a notice that there is a kernel upgrade available - so I upgraded from 5.10.151-1 to 5.15.76-1 but I don’t know ANYTHING about which headers to install or how to go about it . could someone give me a hand please ? very explicitly .
Why do you think you need the kernel headers to begin with? You only need them if you have a DKMS module installed.
FYI, both 5.10 and 5.15 are LTS (Long Term Support) kernels, you can use either. If you do actually need the headers, it’s fairly obvious which to install–the version matching your kernel. 
Yochanan
I’m the poorest Linux user you’ve ever encountered. partly because I don’t remember well anymore. but I refuse to use windows. So , to answer your question . I don’t know what a DKMS nodule is. I thought I should install appropriate headers because I have seen warnings, etc. , about headers and kernels going together; in various places , sites, dialogues, forums I have seen this ; so I thought I needed to make sure .
Have you asked for a warranty replacement for your noodle? No one every gets back to me. 
In all seriousness, I also have a terrible memory as well.
Then you most likely do not need the headers at all.
What’s the output of:
pacman -Qs dkms
If there are no packages ending in -dkms, then you do not need the headers.
See Dynamic Kernel Module Support - ArchWiki
Please share them with the rest of the class.
Sure, nothing wrong with that.