Please someone assist me with chmod, I have the following file structure

-dir1

--file1

--file2

--dir1a

---file1a1

---file1a2

--dir1b

---file1b1

---file1b2

How do I chmod 655 to all files under dir1 and all subdir?
So all files will have 655 permission and all dir will remain as they were.

Thank you