how to delete a primary key which is having auto increment

Question

i tried two codes to delete primary key,here am posting my two codes.please help me.

    1.    ALTER TABLE User1
   DROP CONSTRAINT PK__Users__1788CCACD82F68B5

  2.ALTER TABLE PK__Users__1788CCACD82F68B5 DROP PRIMARY KEY;
these was error for the first code //Could not drop constraint. See previous errors.
 these was the error for second ode//ALTER TABLE PK__Users__1788CCACD82F68B5 DROP PRIMARY KEY;