Is there a way to overwrite the existing term set or will I only be able to add new terms to it manually? I have a csv file I used to import a term set. I recently added terms to that csv file and now want to update the term set with those new terms. However, when I upload the term set again, it doesn't overwrite the existing one, but rather creates a new one with a '1' appended to the name. Removing the old term set and adding the updated one with the same name does not work.
The Managed Metadata Service, which houses the term store (Groups, TermSets, Terms), is a farm-wide service. Any PowerShell scripts or .Net SharePoint API code that modifies any of these must be executed on one of the servers in the farm, whether it's a Console Application, Windows Form, Webpart, Application (ASPX) page, etc.
Overwriting (i.e. deleting and recreating) a TermSet can be a dangerous operation: the Guids behind the terms that actually associate site columns to Terms/TermSets will be destroyed, causing any existing site columns that are bound to the TermSet or any Terms therein to become dissociated. Bad news.
Although it is possible to preserve these Guids by various means, the code to do so isn't trivial, and I strongly suspect there's a better solution to your root problem.
If you've read this far, can I ask why you need to change the TermSet's Terms so drastically that you need to overwrite the entire TermSet?
you could use something like this
Another suggestion is sign DEPRECATED the old terms