Not necessarily recommended but, insert a copy of the row where you want to change the number, but with the ID you require:

SET IDENTITY_INSERT aTable ON
GO

-- Attempt to insert an explicit ID value of 3
INSERT INTO aTable (id, product) VALUES(3, 'blah')       
GO

SET IDENTITY_INSERT aTable OFF
GO

Then delete the row with the number you don't want (after you update any FK references).

More details here: http://technet.microsoft.com/en-us/library/aa259221(v=sql.80).aspx

For posterity, to clarify the question in the comment below, the auto increment value will only be affected if you insert a value greater than the current maximum.

Quoting from linked article:
If the value inserted is larger than the current identity value for the table, SQL Server automatically uses the new inserted value as the current identity value.

Do not do this!

What you are trying to do is actually a SQL antipattern.

Even a deleted ID contains information (i.e., that a record has been deleted), and you should therefore not reuse IDs. Moreover, if the old ID is still referenced in another table (or in the notebook of the old-fashioned guy next door), troubled waters are ahead.

I highly recommended the book SQL Antipatterns . It contains a more detailed description of this antipattern (as well as lots of others). Note: I am not affiliated with the authors of that book.