This works fine to change an input:

document.theform.f10.value = 'test';

But the input that gets changed needs to be based on a variable in the function. So, it needs to work like this:

var input = "f10";
document.theform.input.value = 'test';

But I can't get it to work.

Any advice?

You could always use the getElementByID() function. Add an ID value to your input field
(<input id="some_id" ... />), and access it by

getElementByID('some_id').value = 'test';

// or if you need to access it several times
var elem = getElementByID('some_id');
elem.value = 'test';

I think you mean something like this...

var input = "f10";
document.theform[input].value = 'test';