NO.. no.. no...

Sorry to be so blunt, but upgrading is a little more complex.

Firstly, you need to understand that over the last few years since you installed ZC, you have probably made CUSTOM edits to certain files.

If you were a good little zenner, you would have followed the OVER-RIDE principle religiously.

If you ever installed additional modules, you would now be referring to the WRITTEN NOTES YOU TOOK, indicating what files replaced or over-ride any CORE files.

Then... when you have a full LIST of all the files that were ever customized and/or loaded as part of an installed module, you will now COMPARE these files with their CORE counterparts from 1.3.9c.

You will (using your knowledge of PHP) then decide what to edit into your customized files, based on what has been changed in their core counterpart.

Only when ALL your CUSTOM files (and module add-on files) are consistent with the 1,3,9c schema, can you safely attempt an upgrade...

... and there is a tutorial for doing that.