Yes, both of the drives are upgradable.
One you've replaced one or both of the drives, follow the instruction below.
Download the recovery image for Microsoft Surface from Microsoft’s website
Format your flash drive in FAT32 format and extract the content of the zipped file to the flash drive.
Step 1: Shut down your Surface.
Step 2: Insert the bootable USB drive into the USB port on your Surface.
Step 3: Press and hold the volume-down button on Surface.
Step 4: While holding down the volume-down button, press and release the power button.
Step 5: Follow the on-screen instructions to boot from your USB drive.
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