So here is my situation: I want to upgrade my MacBook Pro mid 2010 to Samsung ssd 850 Evo. I used carbon copy cloner to clone my Mac to an external drive (WD My passport for Mac). Then I installed the Samsung ssd in my Mac. And now I'm trying to get the data back from the external drive to my newly installed ssd but having difficulties. What's the best way to do that?
If I am understanding correctly it sounds like he cloned is old internal drive to an external drive, installed an SSD in the Mac and now wants to copy his files back.
If the new drive has Mac OS installed on it (if not probably best to install it. COMMAND-R and select internet recovery) you can just use Migration Assistant to copy your user files and (optionally) apps to the internal drive. Run Migration assistant and it will have an option for restoring from a local drive. Just select your CCC backup.
If the new drive has nothing on it, reboot the Mac while holding OPTION and select the external drive to boot from. Then use CCC to clone the boot drive to the internal drive. Doing it this way you won't get your recovery partition and might end up with other boot issues. Go back and use the first procedure.
I am assuming you are just turning on the Macbook? Have you tried holding down the Option key as stated here: https://support.apple.com/en-us/HT204417 and choosing the drive to boot from in the Startup Manager? And do you have an external drive also plugged in? Or just the new one? The computer may be trying to find the old boot drive? You don't say what is happening.
As stated in the comment you did not provide us with what the difficulties are.
After it boots the first time you can set the default startup disk. I would have preferred to use a comment for this but I don't have enough rep for that.