Or any other way where I can easily find one of my older posts to make an edit. Only way I've seen to do it is from the dashboard.
Found it: on the right side in your dashboard under yourtumblrname.tumblr.com it lists the # of posts you have -- click that and it'll filter.
There is a link on the left with X number of posts. You will find the mass editor there.
As of 5/2019 Click lightning bolt icon to open the Activity panel. On the right side, at the bottom of the list there in small text it says "Mass Post Editor" That's what you're looking for now.