Pricing for SSD are constantly dropping to upgrading your primary disk partition (C: drive) will be common place. Here are the steps I took to upgrade my 128GB to 500GB SSD.
First you will need a recover disk. In Windows 8.1 search for “Create a recovery drive”. I used a 32GB USB flash drive, though 16GB would have sufficed.
Second you need to create an system image of your primary drive. To do this follow the steps outlined here:
http://winsupersite.com/windows-8/windows-81-tip-use-system-image-backup. I created my image on a secondary slower SATA drive. Once complete you should see a WindowsImageBackup folder in the destination. This should work the same way in Windows 10.
Now you are done with the prep work, go ahead and boot off the USB key and go into Troubleshoot and Advanced to do a system restore. It will automatically find that system image backup created in step 2.
Once you restore your primary drive will be the same size as the original, so the last step is to extend the drive which you will need this excellent free tool: http://www.partitionwizard.com/free-partition-manager.html
That’s it and good luck