Update Piwik 0.2.37 > 0.4

is it really enough to overwrite the old files or is it better to delete the old files first? I got some problems in den backend “widget not found”.

And is it important or better to change the tracking code with this new version?

Installing into a new folder is the safest thing to do, as it is independent of what version you’re updating from. Depending on what version you’re updating from, overwriting could result in a mix of old and new files.

Legacy, pre-0.4 tracking code is supported by 0.4, for backwards compatibility.