Step 1: Separate structure commands from data commands.
SQL commands split broadly into DDL, which shapes the database's structure like tables and indexes, and DML, which manipulates the actual rows of data stored inside that structure.
Step 2: Sort the given options into these two buckets.
ALTER changes a table's structure, DROP removes a database object entirely, and TRUNCATE wipes all rows from a table in a way that is treated as a structural operation because it cannot easily be rolled back, all three belong to DDL.
Step 3: Look closely at UPDATE.
UPDATE does not touch the table's structure at all, it only changes the values stored in existing rows, which is the very definition of a data manipulation command, placing it firmly in DML alongside INSERT and DELETE.
Step 4: Conclude.
The command that is not DDL is
\[ \boxed{\text{UPDATE}} \]