ALTER TABLE `project_id.dataset_id.table_id`
ADD COLUMN new_column_name STRING;

ALTER TABLE `project_id.dataset_id.table_id`
DROP COLUMN column_name;

ALTER TABLE `project_id.dataset_id.table_id`
RENAME COLUMN old_column_name TO new_column_name;

ALTER TABLE `project_id.dataset_id.table_id`
ALTER COLUMN column_name SET DATA TYPE INT64;

CREATE OR REPLACE TABLE `project_id.dataset_id.table_id` AS
SELECT
  column1,
  column2,
  new_column_name AS new_column_alias
FROM
  `project_id.dataset_id.table_id`;