As we all are eagearly waiting for Terraform 0.12 official release, there might be few issues when trying now to update to that version from previous one.
If you are getting errors such as:
"Error: Unsupported block type Error: Missing newline after argument"

.. I would suggest that you run "$terraform fmt" with previous TF version (e.g. 0.10.7)
That should fix syntax for TF 0.12

If you are getting error:
"The variable name "count" is reserved due to its special meaning inside module blocks."
... I would suggest that you rename "count" variable as it's now reserved word.

If you are getting error:
"Error: Failed to download module"
... I would suggest you check if you have added "./dir_name" into your module path.