There's no need to change eBay's estimated weight; just ignore it and enter your actual weight in the boxes provided.
Are you having trouble choosing MM as a shipping service? If it's not offered in the list of "Economy" services, you are probably listing in one of the categories which eBay has decided aren't eligible.
Sometimes, eligible items are affected (such as putting a book about crafts in the Collectibles category). In that case, the workaround would be to enter a flat rate for generic "Economy" shipping, based on the cost for Media Mail. You would also not be able to purchase MM postage through eBay for such a listing, but it's available through other online postage sellers and also at your Post Office counter, where the price is the same as through eBay.
