It is really important that if you are using a delete query you have made a copy or backed up your data. An Access delete query will delete data based on criteria you set. You cannot undo it. It is good, however, for removing old data that is no longer required, although there is usually a backup of it somewhere.
Data in databases sometimes require updating, perhaps it is a product name that has changed and needs to be changed in many places or maybe it is a change to the system of stock codes, area codes or perhaps a change to costs of a specific product. In Microsoft Access you can do this using an update query.