How do I make one field equal to another?

How do I make one field equal to another?

 How do I make one field equal to another? I know this must be simple, but it's eluding me tonight.

Field 1 is an email entry field

& Field 2 contains a default email address specified as it's initial value

I would like user input into field 1 to change field 2 away from the default to the new value.

Any help with this would be hugely appreciated...

Thanks everybody

Dusty