And I'll add to this ^ that macOS also can't mount/read that second ext4 partition.
If you can get your hands on an Ubuntu Live CD or USB you can boot up your workstation so that you can edit files on that second partition.
But that suggestion to use Notepad for this since is a moot point. Use nano (command line) or gedit (GUI).
Take the SD card out, put it in a card reader. Create a file octopi-password.txt on it, containing a new password as it's only line. Unmount, boot, login with the new password.
Put that in ages ago for cases liked this and for easier provisioning. Should hopefully still work.
This still works! Such a great idea, there was no way i was going to remember the password, and the init=/bin/sh method didn't work for me so this was perfect. Great foresight!
Worked for me also. Without this I would have had to restore from one of the images but which one?
I had changed the pw for security and wrote it down and taped it to the pi (as I do with all my devices) but somehow it stopped working after a couple of years.
An important safeguard for all releases. Thanks from a sometimes user and Patreon-level supporter.
Where do I put this file?
For me, since I have macOS, I connect the microSD into the SD slot and it might look like this in a Terminal session:
cd /Volumes/boot
ls config.txt # Just to make sure I'm in the correct place
sudo echo "mypassword" > ./octopi-password.txt
Thank you for your help.
Brilliant ! cheers for this 
Does this reset the username? I can't remember that but I do have acsess to the SD card
Any way to set the username and password via the card because on losing my login info I've also lost the login info for SSHing to the pi. So I can't do the reset
The default username is "pi" and I'm guessing that this octopi-password.txt trick will reset the password on that account (only). If you have created other usernames, you should be able to use "pi" to figure out what those names are.
This worked perfectly for me thanks
Just chiming in to say that this worked perfectly for me. I had to relocate my printer, and then of course had forgotten the password.
This fix is simply the best thing that's happened to me all month!
Type in SSH pi@[network IP] and then enter the pw you specified in file when prompted. (I figured this out on my own, lol)
works "today" (and before my last OS update - Octoprint was already: 0.15.1 / 1.6.1), but you should explain also that
Same claim is for the file wpa_supplicant.conf, but this is a raspberry topic and not a top security theme
Wow still works, what a life saver. That would have really sucked
{ Take the SD card out, put it in a card reader. Create a file octopi-password.txt on it, containing a new password as it's only line. Unmount, boot, login with the new password.
Put that in ages ago for cases liked this and for easier provisioning. Should hopefully still work.
You absolute LEGEND. Thank you.
Just tried this on a fresh (apart from restoring a config backup) 1.8.0 install. Default 'raspberry' password did not work with the 'pi' account, nor did a password set with the octopi-password.txt tool. Tried with both Window's default SSH command and with putty, access denied on both.
did you change the username in the pi imager?
I re-imaged again, explicitly setting the username to pi (rather than leaving the option unchecked in the imager, which defaults to pi anyway). SSH then worked fine. Restoring the config backup then returns to SSH throwing access denied again. I'm guessing something in the config backup is messing with SSH (I hadn't even needed to use SSH before, so I don't know whether it was broken or not previously), so I'll just reflash and reconfigure from scratch instead.
That's really weird.
The backup contains only OctoPrint related stuff and not touch the OS.