You just need to backup the data on the HDD somewhere else, because you will be initially cloning the SSD onto the HDD and that would delete the data from the HDD
Then you will ne inserting the new SSD and cloning the HDD onto that.
Honestly, this is a complex task and if you have never done something like this before, it would be best to get someone there to help you with this, to ensure you do not loose your data.