Follow the instructions below in order to reset the password in MassMailer.

Step 1: 

Navigate to MassMailer app in Salesforce -> MassMailer Setup -> MassMailer Login

Make a note of the User Name, API Key.

Step 2: 

Reset the password in SendGrid by going to

Step 3:

Make a note of the updated password in the previous step. Now go back to Salesforce. Navigate to the MassMailer app -> MassMailer Setup -> MassMailer Login.

Click on the Edit button; Enter the user name, password, and the API Key and Save.