Since IF EXISTS is not supported in indexing by MySQL, you may want to write your own migration:

def drop_index_if_exists(apps, schema_editor):
    # access to the connection since schema_editor.execute does not return the cursor
    with schema_editor.connection.cursor() as cursor:
       cursor.execute("SHOW INDEX FROM table_name WHERE KEY_NAME = 'index_name'");
       exists = int(cursor.fetchone()) > 0
    # outside with to close the cursor
    if exists:
        schema_editor.execute("CREATE INDEX index_name ON ...")

operations = [
    migrations.RunPython(drop_index_if_exists)
]

For consistency, you can write a create_index_if_not_exists method to un-apply the migration, and call it:

migrations.RunPython(drop_index_if_exists, create_index_if_not_exists)

Here there is one solution, thank you for idea @alfonso.kim

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import migrations


def drop_index_if_exists_and_create(apps, schema_editor):
    # access to the connection since schema_editor.execute does not return the cursor
    with schema_editor.connection.cursor() as cursor:
        cursor.execute("SHOW INDEX FROM table_name WHERE KEY_NAME = 'index_name'")
        exists = True if cursor.fetchone() else False
    # outside with to close the cursor
    if exists:
        schema_editor.execute("DROP INDEX index_name ON table_name")

    schema_editor.execute("CREATE INDEX index_name ON table_name(index_name(191))")


class Migration(migrations.Migration):

    dependencies = [
        ('table_name', ''),
    ]

    operations = [
        migrations.RunPython(drop_index_if_exists_and_create)
    ]

I'm getting a Transaction Error after executing your code:

django.db.transaction.TransactionManagementError: Executing DDL statements while in a transaction on databases that can't perform a rollback is prohibited.

Django 2.5 MySql 5.7