you need an update query
Id suggest using the query designer and pulling up the update option
generally speaking you are looking for something like...
the where claues is important, you need to make certain that you limit the update to the required records. I'd strongly suggest that if you are developing an update query, make sure you don't do it anywhere near live data... make sure you are running on a development db, or back up table.
dateadd is almost certainly the "right" way to add a day. however if you are using the default data storage mechanism in Access then adding one to a datetime value has the same effect. (adding 1/24 adds an hour, subtract 1/8600 subtracts one second [1/(60 seconds*60 minutes *24 hours)]