Replacing nested foreach with LINQ; modify and update a property deep within

foreach(var item in company.internalData.Emps
                        .SelectMany(emp => emp.privateData.Addresses)
                        .SelectMany(addr => addr.Items)
                        .Where(addr => addr.Type == "StreetAddress"))
     item.Text = CleanStreetAddressLine(item.Text);

LINQ is not intended to modify sets of objects. You wouldn't expect a SELECT sql statement to modify the values of the rows being selected, would you? It helps to remember what LINQ stands for - Language INtegrated Query. Modifying objects within a linq query is, IMHO, an anti-pattern.

Stan R.'s answer would be a better solution using a foreach loop, I think.

var dirtyAddresses = company.internalData.Emps.SelectMany( x => x.privateData.Addresses )
                                              .SelectMany(y => y.Items)
                                              .Where( z => z.Type == "StreetAddress");

  foreach(var addr in dirtyAddresses)
    addr.Text = CleanStreetAddressLine(addr.Text);