Here is a quick example, i'll walk you through..
1 Lets create a table called UserTable
2 insert four rows
3 delete the first row (so we are where you are now, a table that starts from ID=2
My solution: copy the contents of the table to a temporary table, truncate the origional table and load it back from the temporary table
CAUTION: if the table has relationship (or if the ID that you are changing is foreign key on other table), you have to consider updating all related tables as well.
CREATE TABLE [UserTable](
[ID] [int] IDENTITY(1,1) NOT NULL,
[Name] [varchar](50) NOT NULL,
[UserAddress] [varchar](50) NULL,
CONSTRAINT [PK_UserTable] PRIMARY KEY CLUSTERED
([ID] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]
GO
INSERT INTO [UserTable] ([Name] ,[UserAddress])
VALUES ('John' ,'Address 1'),('Mary' ,'Address 2'),('Doe' ,'Address 3'),('Kent' ,'Address 4')
GO
Delete From [UserTable] where ID = 1;
SELECT [Name],[UserAddress] INTO #temp from [UserTable];
Truncate Table [UserTable];
INSERT INTO [UserTable]([Name],[UserAddress])
SELECT [Name],[UserAddress] FROM #temp