Changing an Item in the Masters does not affect any Bid where the Item is already used (in a Condition) although changes to a Bid-Level Item can be saved to the Masters, if you have sufficient database rights. See Related articles for more information on Master v. Bid-level Items.
You may be asking, "Where is the Bid Item List", well, that is split between the Materials Tab and the Labor Tab. There is no "Bid Item List" comparable to the Master Item List.
See Related articles for more information on Master v. Bid-level Items as well as a refresher on the Master Items List.