I need to install the latest Mac OS Big Sur onto my laptop but it says I haven't got enough space on my hard disc and I need a further 7.9 Gb. What can I do so that I can install?

How to free up storage space on your Mac - Apple Support

It is clear that you have gotten to the point that you need more storage. The easy solution is to purchase an external drive that you can transfer files onto. They are now very inexpensive, available in all sizes, easy to install with a USB cable and most do not require external power. You might have to reformat it, though. Then, problem solved.

As an aside, you will need more than that because you should always maintain a minimum of 15 - 20 GB of empty space for the OS to function properly. If you allow it to fill up, your computer will stop functioning.