Backup SQL Server database on UNC path using SA We need to backup some database to an unc path, the problem it's that it's a SQL Server instance that has to execute with SQL Server autentication (sa user). 

How can we configure the plan task in SQL Server to make it able to have access to the UNC folder? I attach the state of our SQL Server Services in SQL Server Configuration Manager.

Thanks in advance and regards,

Pedro Sánchez.
You can map a local drive letter to your UNC folder using xp_cmdshell like:
exec xp_cmdshell 'net use X: \MyServer\MySharedFolder\'

Then you can issue your backup statement to use the X: drive as if it was local.

You may have to run sp_configure to enable xp_cmdshell.