Converting button to mouse click

can someone please help me change this to a left click on the mouse button. Thankyou

if(Input.GetButtonDown("Jump"))

By default, the correct code would be:

if (Input.GetButtonDown("Fire1"))

That's if you didn't change any values in your project settings. More info on that function HERE.

Alternatively, you can also use the following code to process anything when the left (0), right (1), or middle (2) mouse button is "pressed":

if (Input.GetMouseButtonDown(0))

A bit more info on that function HERE.