Add 'Other Amount' to Amount box

I would like to add the words other amount to the blank input box in the Donation Information section (see image)
I have tried to look through all the various files but have been unable to do so
Please can you tell me how to do this
I am guessing it is a simple line to be added to the bottom of the default.php file
But sadly I do not know php well enough to do this!

Thanks