New indexes are automatically given system-defined names based on the database table name. If you create multiple indexes on a table, the index names are appended with "_1", "_2", and so on. You can rename an index as long as index names are unique within the table.
Note When you create a primary key or unique constraint for a table, an index with the same name as the constraint is automatically created for the table. Because index names must be unique for a table, you cannot create or rename an index to have the same name as the primary key or unique constraint for the table.
To rename an index
The index is renamed in your diagram when you exit the Index name box. It is renamed in the database when you save the table or diagram.