How to modify a foreign key constraint in MariaDB?

Modifying a foreign key constraint in MariaDB involves two steps: dropping the existing constraint and creating a new one.

Here is the basic syntax for both:

1. Drop a foreign key constraint:
```
ALTER TABLE table_name
DROP FOREIGN KEY constraint_name;
```

1. Add a new foreign key constraint:
```
ALTER TABLE table_name
ADD FOREIGN KEY (column_name)
REFERENCES referenced_table_name (referenced_column_name);
```

Here, `table_name` and `column_name` are the names of the table and column where the foreign key currently exists, `constraint_name` is the name of the foreign key constraint, and `referenced_table_name` and `referenced_column_name` are the table and column that `column_name` references to.

NOTE: Be careful when modifying a foreign key constraint. When a foreign key constraint is dropped, the integrity between the related data in the two tables is no longer enforced.

Also, when you add a foreign key constraint, all the existing rows must satisfy the new constraint. If not, you’ll get an error and the table will remain unchanged.