Jay
You are using a windows machine with the drive connected, on which you flashed batocera on, right?
If you use WinRAR or 7zip it should be possible to just right-click drag and drop on the downloaded boot.tar.xz-file to the boot-partition of the drive, where you installed/flashed batocera on and then select “extract here”.
You can replace everything (but you should backup the batocera-boot.conf file before, if you have made changes to it (the wifi-password (if you used wifi on batocera) is saved into this file for example)).
Jay I cant unzip it as my pc just freezes
I guess you are double-clicking on the boot.tar.xz file? This is normal then, that it takes a while until WinRAR (or 7Zip) opens this file. It is the same for me (takes up to 20 seconds, even though i have a powerful machine with intel core i7-4790k and ssd). I don’t know why it is like that.