How can I upgrade my 128 GB MacBook Pro w/retina display to have more than 256 gb? Will the process erase all my stuff?

Depending on the exact model of Macbook you have, you may or may not be able to upgrade storage space.

The upgrade requires replacing the "hard drive" or storage medium entirely, so to not lose any data, files or information, you would need to have a full backup made prior to the upgrade.

If your Mac can be upgraded you would need to purchase a new larger hard drive of the size you require and install it in your computer in place of the existing one. Apple will not aid you in this endeavor however, and opening the Mac to install the drive will void its warranty if not done by Apple or an Authorized Service provider.

You can view potential upgrades if any are available for your specific model of Mac here: SSDs (Solid-State Drives) Designed for the MacBook Pro

Apple never really designed any of the Retina MBPs for upgrades. The drives are either some proprietary interface or even permanently part of the logic board. Apple won't do anything other than replace a failing SSD with the exact capacity. There are a few models that can be upgraded, but it can be a pretty complex process. The only aftermarket drives are sold by OWC or Transcend. They also come in oddball capacities such as 240/480/960 GB.

Total solutions to upgrade your Mac

Which specific model do you have?

If you're looking to upgrade you don't necessarily have to erase everything. You could do things a few different ways, such as clone your boot drive or back it up to Time Machine and restore that to your new drive.

It's kind of complicated. All these SSDs are proprietary since they went to the Retina MBP models.

https://beetstech.com/blog/apple-proprietary-ssd-ultimate-guide-to-specs-and-upg rades

There are guides for how to do it. iFixit refers to the process as "moderate" in difficulty. It requires specialty tools that you might be able to find at some electronics stores. The hardest part seems to be removing the case.

You can buy an OEM SSD used or even new, but I'm not sure where anyone selling them obtains a new one. There are also the aftermarket ones. Some people claim to be able to use industry standard SSDs (some using adapters) but I'm rather skeptical. That's really more of a serious hobbyist's task to get it to work.

Also - Apple doesn't specifically support DIY replacement. If you choose to do it yourself or even have a shop do it for you, it's likely that Apple will consider it to be "tampered" and ineligible for any servicing - including a battery replacement service. That's something worth considering before proceeding.

Just one more thing. If you're really thinking of doing something like this, then it might be easier to simply use an external drive. Possibly a large external drive for less speed critical data such as photos/video. Maybe an external SSD as an external boot drive, which can be reasonably fast. It might be a little bit more complicated to install BootCamp on an external drive, although there are tutorials on how to do it.

Replacing the internal SSD comes with a lot of risks - that it won't work, that Apple or an AASP will refuse to work on it, etc. There's a lot that you can do with an external drive.

128 GB is generally enough for an operating system and lots of applications. It may not be enough for someone with a large photo/video/music library. Those aren't as speed critical and of course Apple's internal SSDs are expensive. And Apple makes it easy to boot externally.

A 2015 MacBook Pro can be updated with a larger hard drive. As explained in the link for macsales.com a posted above offers several sizes for the SSD replacement, tools, and a vide guide.

Look through that, and the prices there before making a decision.