Since the Aspire One D255E is a Netbook, I'm pretty sure it does NOT have space or connections for an extra hard drive. You will need to remove the current drive, remove the caddy and any adapter that may be plugged into/onto the SATA connections (some have them, some don't), then install the SSD drive in the caddy with any associated adapter.
If you need to transfer everything from the old drive to the new one, then that's a whole new side of the operation. Acronis, Minitool and others have free versions of disk cloning software available, but you'll need to put BOTH drives in another machine or use an external adapter to run one from a USB port in order to do this. Many netbooks do not have optical drives, so it is usually best to utilize a separate computer entirely to do the cloning.
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