All your custom and union tables can be managed from the search window, selecting selecting New → Custom table → Manage.

my.blend deprecated

Even though it is no longer possible to create custom tables with the prefix my.blend, those that already exist will appear in the manage window so you can handle them.

To edit one of them, you just need to locate it on the table list and click it.

In the left area, you can see the table name at the top and the Table fields section below it, which displays all columns contained in the table. You can delete the whole table by clicking the trashcan icon next to the table name or delete specific columns by clicking the trashcan icon of the corresponding column in the Table fields area.

In the right area, you can see the queries used to feed this table and, if you click the expandable next to the query name, the columns involved. You can remove one of the queries by clicking the trashcan icon next to the query name or one of the columns by clicking the trashcan icon next to the corresponding column.

What permissions do I need?

Managing tables