Go into BIOS. Set it as first boot device? All the new dells these days they aren't like
First Boot - hard drive
Second - DVD DRive
and then you have to select what is the first HDD and first Optial etc. If you go into the BIOS and go into Boot it list ALL the devices and you just use the up and down arrows on the right to select it.
ALSO if you do NOT have an OS on the 1TB, and have the 256GB in there it will still boot from the SSD even if it is the last boot device so long as the previous devices DON"T have anything to boot from.
Is the OS installe don the 128 GB or no? if it is you can just either 1) Swap it out, then reinstall windows with a Dell Windows 10 disk or even a regular windows 10 disk or 2) You can use a program like Macrium Reflect, you first power on the laptop, go though setup, install the program, CREATE A RECOVERY DVD/USB DRIVE, then make a Image of the 128 and save it to the 1Tb. Then replace the 128 with the 256, boot off the disk/USB drive, browse to the image and restore it to the 256. You will have to extend the partition though to fill up the full 256 GB though but that is easy.