SQL is a pretty simple language to understand. The above statement basically means
UPDATE <table_name> SET <what_to_change> WHERE <change_criteria>
So we’re updating the wp_posts table. We’re setting the value of post_type in that table to ‘post’ (it is currently ‘page’). And we’re making that change for the post record identified with an ID of 170 (or whatever your post ID is).
Each record in that table is equivalent to a post of some type in your website, be it a post, page, revision, attachment, etc. We want to change from page to post. All records are identified by an incrementing ID number. That’s how we specify which record to update (you can update multiple records at a time). We found the ID from the URLs.
That’s how we come up with our UPDATE statement. Pretty simple once you know how.
FYI: If you left off the WHERE statement at the end, you are no longer filtering for certain records, and you would update EVERY record in that table. This is not good. It would mean that all the revisions in there (auto-saves) just became live posts ?? Type carefully. Little mistakes can cause big problems