MCoale

@darsen as casuallyglam mentioned, do a full backup if possible. I was personally unable to access my dashboard/backend, so I manually downloaded my whole wp site to my local hard-drive. This may have been overkill, but I found it necessary, as I didn’t want to lose everything.
After that, I manually copied the 4.2 files, overwriting the site. Doing this didn’t affect any posts or data on my site.
NOTE: try to only copy the contents of the folder where you’re getting the error messages for first, then branch out.

All the content should already be saved to a database on your server, so you should be fine doing this, however, as I mentioned, it’s wise to backup first! Hope this helps.