chscag is RIGHT! (in reply 29 above)
DON'T trust anyone else to do this.
Do it yourself.
It will take 15 minutes of time and a Phillips #00 driver and a TORX T-6 driver.
Most of the time is involved taking the screws out of the back cover and putting them back in.
The more involved part is getting THE CONTENTS of your drive "migrated over" from the old drive to the new.
I would just use CarbonCopyCloner (which is FREE to download and use for 30 days).
I would also buy this little gadget:
https://www.amazon.com/Sabrent-2-5-...478&sr=1-2-spell&keywords=sabremt+usb3+to+ssd
Only $12.50 and VERY useful.
Use it to connect the SSD BEFORE you open the back to change drives.
Use Disk Utility to initialize it to Mac OS extended with journaling enabled.
Now the SSD is ready to "clone over" your stuff from the old (still internal) HDD.
When the clone is done, REBOOT the Mac.
Hold down the option key IMMEDIATELY after the startup sound, and KEEP HOLDING IT DOWN until the startup manager appears.
Select the [external] SSD with the pointer and hit return.
Do you now get a good boot to the SSD?
If you used CCC it will look EXACTLY LIKE THE INTERNAL DRIVE.
You must use "about this Mac" to see if you're really booted to the external SSD.
If it boots right up and looks good, now it's time to do the drive swap.
Once the swap is done, and the back is on, reboot the Mac -- AGAIN with the option key, and select the SSD as the NEW internal drive.
Do you get a good boot?
Fine, now open startup disk preference pane and RE-set it to be the boot drive.
Now you're done.
Use the USB3/SATA dongle/adapter to access your old drive.
Don't throw it out as long as it still runs.
You can keep it as a backup, or for extra storage.