How to change collation of database, table, column?

Changing it database wide:

ALTER DATABASE CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

Changing it per table:

ALTER TABLE CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

Good practice is to change it at table level as it’ll change it for columns as well. Changing for specific column is for any specific case.

Changing collation for a specific column:

ALTER TABLE MODIFY VARCHAR(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;