The service account needs to be changed or the the password for the existing service account needs to be updated.
There are two locations where the service account can be applied:
Backup Schedule Account
The scheduled backup account is the account that runs the schedule task within the Windows Task Scheduler. It is the account that starts the backup process, if no account is specified to access the backup agent (as noted in the below section) then it is also the account that copies the backup files to the domain controller and generates the backup file. Follow the below steps to update the backup schedule account:
Backup Agent Account
An account can be specified to access the backup agent. If this account being used then the above schedule account is only used to start the scheduled task. Follow the below steps to update the backup agent account: