More details:

Before I updated the platform, I upgraded a plugin - I think Newsfeed. That was probably not wise. When I then tried to upgrade the platform, it didn't work. So I upgraded the platform via the manual method of unpacking the package to my root and loading the /ow_updates link. Now I'm seeing my Newsfeed box with no button to post, and under that box the above error. Under that error, I'm seeing this separate one:

Message:Trying to get property of non-objectFile:/public_html/mysite/ow_smarty/template_c/6126f863f18d2a7f456dfff404fcc790b5d7432b.file.attachment.html.phpLine:26

Joshua, you need to update the platform first, then plugins. Do you have a backup?

Mostly. I backed up all the files except for some of the /ow_smarty files because they were taking too long. 

When I switch back to the old files before any of the updates, I'm getting this on the admin page:

OW Debug - WarningMessage:Can't generate URI! Route `mobile.admin_settings` not found!File:/ow_core/router.phpLine:262

As well as a bunch of other errors on the index page

I've got the old backup reloaded and it's not telling me that a new release is available. Plus I'm getting errors still. Anyone know the best way to proceed?

I changed the ow_plugins/newsfeed folder to newsfeed28 and that got rid of the errors. But I still have lost all posts and there's no Post button to add new stuff via the newsfeed?

Joshua, you will still need to use your back up, or downgrade your newsfeed plugin version. Then you will need to enable debug again in order to catch the error

if it is the one you get earlier: 

Undefined index: previewIdFile:/public_html/mysite/ow_smarty/template_c/6126f863f18d2a7f456dfff404fcc790b5d7432b.file.attachment.html.phpLine:26

or this one: 

Message:Trying to get property of non-objectFile:/public_html/mysite/ow_smarty/template_c/6126f863f18d2a7f456dfff404fcc790b5d7432b.file.attachment.html.phpLine:26

you will need to enable dev mode the refresh the website

Thank you, Ross.