Can you edit the original post of a thread?

Click on the box at lower left with a pencil in it

Your original post will appear

Edit

Click on the "post message" button

"I just want to put the words "price drop" in the title for the thread/original post."

You can change the title of the original post, you can't change the title of the thread. Those are two separate things.

As above…

What you cannot never change: thread title
What you can change: post title and body

When you start a post, you fill in the "Subject" box. This becomes the thread title and post title. These two titles start off one and the same.

After you posted, you can go back and edit the post title and body as many times as you want — but not the initial thread title.

The unchangeable tread title shows up on the branch's listing of threads. The changeable post title shows up in the specific post itself.