DBCC CHECKIDENT can reseed (reset) the identity value of the table. For example, YourTable has 25 rows with 25 as last identity. If we want next record to have identity as 35 we need to run following T SQL script in Query Analyzer.
DBCC CHECKIDENT (yourtable, reseed, 34)
If table has to start with an identity of 1 with the next insert then table should be reseeded with the identity to 0. If identity seed is set below values that currently are in table, it will violate the uniqueness constraint as soon as the values start to duplicate and will generate error.
Reference : Pinal Dave (http://blog.SQLAuthority.com)