Well, let's do the 'CS' thing, and find out what's going on ...
... who would like to have the honors (since it's never happened to me) to find out?
I have a feeling that you can edit a bid to lower your bid or quantity ... but are not able to increase your bid or quantity.
In the beginning, they use to let you cancel your bid in the first 15 minutes, but it was found out tbat there were folks here who would bid up lots that they wanted, just to see how high their compititions autobid was set, and if it was too high, they would cancel their bid ... so that the original bidder was stuck paying this highest bid ...
... it takes all kinds, I guess.