To delete a relationship, just click it in the Relationships window (the line between the tables turns bold to indicate that it is selected), and then press Delete. Access asks for confirmation; click Yes, and the relationship disappears.
If you delete relationships between tables, you are affecting how information in the tables can be combined in a query, form, or report. It is a good practice to design your tables so that they can be related. Remember that each table is supposed to hold a subset of the database information. If each table is set up correctly, it should have at least one other table in the database that it can be related to.