Help set environmental variable

[Nick Croft <nicko acay com au>]

Greetings!
I'm wanting to save over a .xls file. The dialog warns that this is 
disabled, and goes on to say

        "You can turn this safety feature off by setting the
        environmental variable GNUMERIC_ENABLE_XL_OVERWRITE".

I'm using bash. Would some kind soul please help me cast the right 
spell to achieve this.

Nick