I was NOT able to make a spreadsheet modify with igoogdocs. Don't buy it!
I can modify in iMailG on my iphone (iphone 4 with updated software). And it's free! I almost bought a more expensive one called GoDocs, but once I got iMailG to work, I stopped there.
It's not super fast or efficient, but it works for putting in one value. Click edit in the left column; little boxes appear; type your value; click submit.