How can we convert multi-input NOR gate diagram to a 2-input NOR gate diagram?
convert multi-input NOR gate diagram to a 2-input NOR gate diagram?

This is not a feature of any schematic drawing package that I am aware of.  The answer is therefore probably "you can't".  You have to delete the existing diagram and re-draw it using different gates.

This seems to be especially true since your diagram appears to be ASCII art intead of drawn with something that understands the underlying logic functions.

Just re-draw it.