Can this overwrite previous posts

No, there is no such option in free version. In our pro version file manager and version control helps users to update a exiting csv file and related records.

Please elaborate on how you can update the existing records?
What needs to be used as a reference Value? If I just want to update the content, how do I go about it?

WHat I need is to be able to update content of existing product listing based on a Part Number (Not Description i.e. Post Title). If I create a taxonomy which will have Part Numbers, will I be able to update content of the relevant Products?

I’ll purchase the pro version if you can assist

In pro version all imported csv files are managed through a file manager with version control. Files with same name are considered as an update and saved as incremental versions. You can download, edit and update any version.

Updates are done based on post ids for a existing file version. So you need to maintain the same file name or download the correct file version using fie manager where you can add and edit the content to update.

For example you can rotate the price, offer, discounts etc periodically and set back to older when the offer sale period is finished.