[split] Change password from external media

[James Good]

New (?) question: how or where would I be able to find the file mentioned?  The PBP has Manjaro as the preinstalled operating system and my main computer, a desktop, has Ubuntu 18.04.4 LTS.  What size of SD card might be required?
Thank you,
James Good

I wouldn't use your Ubuntu machine for this. Boot up a live img of Manjaro, on an SD. Identify and mount the emmc root partition, and use chroot to enter the mounted partition. Then you can run 'passwd [user account name here]' in terminal to set a new password for your account. There is no file to find or change.

Only problem with what you suggest; my desktop computer CAN boot and access the Internet, the PBP does NOT fully boot so I am not able to access anything either on the local computer or on the Internet.  What I had suggested was something thai I would be able to do now.  And not having, or knowing where to find, a live img of Manjaro, I am limited to using what is available.
Thank you,
James Good