I am really hoping that someone will take pity on me.
I copy and pasted this command from the internet and then realised I didn’t know how to reverse/reset it.
The idea was to change the behaviour of my mouse temporarily.
This is the command I used:
xmodmap -e "pointer = 4 5 3 1 2
Sorry not in code tags as I am unable to highlight with mouse.
Thank you very much.