SQL Server – How to copy data in the same table based on existing data in SQL?

A simple way to do this would be to first load it into a temporary table, make the changes, and then reinsert it into the main table.

select * into #temptable from table where date = 201839 & # 39;
update #temptable set date = 201840 & # 39;
insert into the table select * from #temptable
update #temptable set date = 201841 & # 39;
insert into the table select * from #temptable