I think you can do it as three different UPDATE statement. I don't have SQL Server at hand to work things out. The sticker will probably be Process #2 which wants a record number. But you might be able to change the schema to get a replacement.

This is a off-the-cuff attempt at process #1 but I'm not sure the math is right.

UPDATE the_table
SET cum = (
 SELECT CASE COUNT(*) 
  WHEN 1 THEN ren 
  ELSE SUM( ((1 + Cumprec) * (1 + Ren)) - 1) END
 FROM the_table t
 WHERE t.year <= the_table.year AND t.month < the_table.month )

-Mike