DBCC CHECKIDENT can reset the identity value of the table.
For example, Your Table has 25 rows with 25 as last identity. If we want next record to have identity as 35, we need to run the following TSQL 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.