In SQL Server, I have a database abc. In this database I have hundreds of tables. Each of these tables is called xyz.table
I want to change
I'm assuming You've already created the schema abc in the database.
If not you can refer here
http://www.youtube.com/watch?v=_DDgv8uek6M
http://www.quackit.com/sql_server/sql_server_2008/tutorial/sql_server_database_schemas.cfm
To change the schema of all the tables in database you can use following system created msforeachtable stored procedure to rename schema of each table with alter schema.
exec sp_MSforeachtable "ALTER SCHEMA abc TRANSFER ? PRINT '? modified' "