What command would you use to change the schema of an existing table in a warehouse?

The command used to change the schema of an existing table in a database warehouse is "ALTER TABLE." This command is a standard SQL statement specifically designed to modify an existing table's structure, which can include adding or deleting columns, changing column types, or renaming columns, among other alterations.

Given that the requirements of schema changes are fundamental in database management, "ALTER TABLE" is recognized across various SQL databases, making it a widely accepted method to make schema-related adjustments. It provides flexibility in managing the database structure without the need to create and populate an entirely new table, ensuring better data management practices.

The other options do not represent standard SQL commands for altering table structures, making them unsuitable for schema modifications.