How to easily convert utf8 tables to utf8mb4 in MySQL 5.5
From my guide How to support full Unicode in MySQL databases, here are the queries you can run to update the charset and collation of a database, a table, or a column:

For each database:

ALTER DATABASE
    database_name
    CHARACTER SET = utf8mb4
    COLLATE = utf8mb4_unicode_ci;


For each table:

ALTER TABLE
    table_name
    CONVERT TO CHARACTER SET utf8mb4
    COLLATE utf8mb4_unicode_ci;


For each column:

ALTER TABLE
    table_name
    CHANGE column_name column_name
    VARCHAR(191)
    CHARACTER SET utf8mb4
    COLLATE utf8mb4_unicode_ci;


(Don’t blindly copy-paste this! The exact statement depends on the column type, maximum length, and other properties. The above line is just an example for a VARCHAR column.)

Note, however, that you cannot fully automate the conversion from utf8 to utf8mb4. As described in step 4 of the abovementioned guide, you’ll need to check the maximum length of columns and index keys, as the number you specify has a different meaning when utf8mb4 is used instead of utf8.

Section 10.1.11 of the MySQL 5.5 Reference Manual has some more information on this.