You CANNOT upgrade to 5.7, see:http://www.concrete5.org/get-started...

You'll need to use a fresh install.

Thanks much for your reply Steevb ...
A fresh install would mean losing all the current content and having to re input it all?

You happen to come across this error when editing?:
An unexpected error occurred. Unable to get permission key for write< Back to Home

There is no upgrade path from 5.6 to 5.7. You essentially have to rebuild your site (Usually not more than a day's work for an experienced coder). There's no need to do this though unless there is a critical add-on you need that is 5.7 only. 5.6.X is still being updated and it's a great CMS. My firm is still launching brand-new sites on 5.6 it as it's more stable, better documented, and has more add ons than 5.7. Our clients also generally prefer the editing interface in 5.6 over the editing interface in 5.7.

Thanks goldfish!
Didn't think of 5.7 that way, but makes a lot of sense.

I hate to keep asking this here, but have you ever come across this editing error:
An unexpected error occurred. Unable to get permission key for write< Back to Home
It doesn't effect editing other than having to refresh the page every time to get rid of it. More annoying than anything, and the client will be looking for a solution. Been searching everywhere for a fix and nothing's surfaced. Only started appearing after updating to 5.6.3.4, and would hate to revert to an older version for it slowed down the sites dramatically.

I haven't seen that error on any of our sites, and I have seen that error mentioned in forums prior to 5.6.3.4. I'd guess a conflict with a custom block or add-on. If you have a backup of your site & database before you upgraded, it would be worth restoring it on a test server to see if the error is gone. If there are no errors, then obviously it's 5.6.3.4 conflict, and I'd stick with 5.6.3.3 in production until you get it figured out.

The site running very slow brought on the update. Since that, the site's working much better. When I noticed the error I did revert back, resolving the errors, but slow site again. Annoying edits are better than a slow site. So here we are stuck with the lesser of two evils.

If something comes to mind later, I'd like to hear it.
Appreciate your replies very much. Hope you have a great day.

To troubleshoot, I'd make a copy of your site on a test server, then start uninstalling / removing add-ons until you find the culprit. You might also try switching back to the default theme to see if it's something in your custom (or purchased) theme that's causing the issue.

Thanks much goldfish, that's a good idea.