<History>
   <Entry>
      .....
   </Entry>
   <Entry>
      .....
   </Entry>
</History>

<History>
  <Entry entryID="AAA">
      .....
   </Entry>
   <Entry entryID="BBB">
      .....
   </Entry>
</History>

declare @eventId varchar(64)
set @eventId =  CONVERT(varchar(64),NEWID())
update Histories
       set XmlHistory.modify('
              insert attribute EntryID {sql:variable("@eventId")}
                     into (History/Entry)[1]
       ')
where HistoryID=285162

select h.id rowPK, m.c.query('.') theElement
from TheTable h
    cross apply h.XMLColumn.nodes('History/Entry[not(@EntryID)]') m(c)
where XMLColumn.exist('(History/Entry)')= 1