If you click on the menu icon in the upper right next to your email address you can switch the site to Advanced Mode
Advanced mode will enable a few extra options in the user interface that can be helpful, but are also potentially dangerous.
In particular with Advanced Mode enabled you can:
edit the amount and quantity of existing bids (see Edit Bid. An edit icon will appear on the Activity tab of the item or bidder details page next to bids. Editing bids in this way does not reapply bidding rules or enforce quantities.
allow access to advanced import features so you can modify item#s and bidder#s directly through an Excel import of previously exported records.