Update query no form numbers in second table

Question

I need to write an Oracle update query to set a reject flag on one table when the Form Number in the second table has all record equal to null.

Update Table1
   set reject = 1

where Table2 has all records where the Form Number = null

I am not a SQL expert and I have to figure this out.  I do not know if I use a Select or Join or what.

I would appreciate any help I can get.
Thanks,
arep