Is it possible to upgrade the HDD to 1TB

The Early 2015 MBP does not have a regular HD. Its a Blade type PCIe SSD. While it is technically possible to change it out, I do not believe there any 1 TB Blade Type PCIe SSD drives out there to replace it with.

https://www.ifixit.com/Store/Mac/MacBook-Air-13-Inch-2015-and-MacBook-Pro-Retina -13-Inch-and-15-Inch-2015-SSD/IF188-108-…

Even opening the case of a 2015 system will void the warranty. You only have what you bought with Retina systems. Phil has countered my "only what you bought" ... but warranty issue remains.

Get an external enclosure and swap 1 TB drives through the enclosure. WARNING ... USB 3.0 connection has been known to cause localized fields that can interfere with BT devices and 2.4 GHz networks.

There are PCIe SSDs that are compatible with your MBP:

http://store.mcetech.com/mm/merchant.mvc?Store_Code=mtos&Screen=CTGY&Category_Co de=PCIEFLASH

Aside from the expense factor, do take into account the admonition that was mentioned by steve359.

I believe Samsung also has a 1 TB PCIe SSD avaliable, but I do not have the link handy.

Ciao.