FOR INVESTIGATION: How do I use the find method?

I know how to use find_by_x. Questions that need answers:

1. How do I use the find method?
2. How do I use the find_by method?
Why do you use the Activerecord where method?

Where is for when you want to get a bunch of

Activerecord where is like picking all of the red skittles out of a bag of skittles.  Each skittle is like a single row in a table in the database. the bag of skittles is like a table in the database.  A red skittle is like a particular kind of record in the database.  The act of picking out all of the red skittles is like using the where command to find a particular kind of record in a table.
Where do I find documentation on the basic update method?
FALIING TEST: How do I use the activerecord update command on a single object?

Sadly, this test fails

Failures:

  1) Activerecord CRUD basic update method #2
     Failure/Error: post.update(title: "hello world")
     NoMethodError:
       private method `update' called for #<Post:0x007fe55b5eef00>
     # ./spec/activerecord_crud_spec.rb:82:in `block (2 levels) in <top (required)>'

Finished in 2.83 seconds
19 examples, 1 failure, 3 pending

Failed examples:

rspec ./spec/activerecord_crud_spec.rb:74 # Activerecord CRUD basic update method #2