To drop a column, first make sure that you are viewing the table that you want to drop the column from.

Next, go to Develop > Table Management. You will see a list of your tables with several options below each. Click the 'edit' option. You will see a form showing the fields in your table, with an options column on the right. Under the options column click the 'delete' link in the row for the column you wish to drop. You will be prompted before you can delete the column.

Be careful! Deleting a column will delete any data that you have entered into the table!