below the list of your "columns" in the table structure you have a small table called "Indexes". Every row is an index. Find there your unique index and drop it using "drop" symbol (red X). If you have it on more than one column then drop it and recreate without the column you don't want unique
You can DROP a unique constraint from a table using phpMyAdmin as requested as shown in the table below. A unique constraint has been placed on the Wingspan field. The name of the constraint is the same as the field name, in this instance.