Follow the instructions below in order to reset the password in MassMailer.
Navigate to MassMailer app in Salesforce -> MassMailer Setup -> MassMailer Login
Make a note of the User Name, API Key.
Reset the password in SendGrid by going to https://app.sendgrid.com/forgot_password
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.