If you need to change the account used by Acronis to log in to vCenter or ESX(i) host in version 5 and 6:

1. In Backup management console navigate to VMware resources.
2. Select the vCenter/host.
3. Click Overview:
   
4. Click on the user's name.
5. Change credentials:
   

If you need to change the account used by Acronis to log in into vCenter or ESX(i) host in version 4:

1. Remove the Agent for VMware.
2. Reinstall the Agent using the new account.