You can change the reserve price on a public auction: click on the "Edit Listing" link on the auction listing.
However, there are some restrictions on the changes you can make.
You can't change the reserve price once it has been met by an accepted bid.
You can lower the reserve price, but you can't set a new reserve price that's lower than the highest accepted bid. That bidder will already have been notified that the bid didn't meet your reserve price, so your new reserve price needs to be higher than that bid.
You can raise the reserve price as long as it hasn't been met yet.