If you mean merging the data from two similar DBs you need to consider the following points
Are you using Autonumber fields. If so you will need to ensure you don't have duplicates between the two versions of each table. You also need to check that any relationships will not be broken if you change the linking IDs
Once you have resolved these issues then you can use an Append query to move the data from one table to another. Or possibly use a Union query. See access help for more info