You will have to drop each index in turn and then recreate it to do this.
ALTER TABLE tablename DROP INDEX indexname;
ALTER TABLE tablename ADD INDEX indexname (field1, field2, ...);
As an example I am using the w3t_AddressBook table:
ALTER TABLE w3t_AddressBook DROP INDEX address_ndx1;
ALTER TABLE w3t_AddressBook ADD INDEX address_ndx1 ( Add_Owner );
www.mysql.com will be your guide and ensure that the ADD INDEX is correct while creating the index because the one that you are working on might be a PRIMARY (and then the syntax changes).
I hope this helps.
