Copy 1 column to another by mysql query

I have mysql database with following table...

| id | amount | tax |
+----+--------+-----+
|  1 | 500    |     |
+----+--------+-----+
|  2 | 100    |     |
+----+--------+-----+

I have to delete amount column but before doing that i want to shift all data of amount row from amount row to tax row. How can it be done using mysql query? Please help.