In mysql based on the condition how to replace the particular value in the comma seperated string code example

Example: fetcht he leftmost word in a comma separated string in sql

SELECT SUBSTRING_INDEX(address, ',', 4) FROM employees;
+-----------+------------------------------------+
| name      | SUBSTRING_INDEX(address, ',', 4)   |
+-----------+------------------------------------+
| John Doe  | 4225 Collins Street,Apt. A,Erie,PA |
| Bob Smith | 234 Main Street,,Erie,PA           |
+-----------+------------------------------------+