Simply unplug the power cable and SATA interface connector from your 3.5 inch hard disk drive inside the chassis of your PC.
Connect both cables to your 2.5 inch SSD. Place the 2.5 inch SSD gently in a location inside the chassis where it will not bounce around or get damaged.
For greater structural integrity, you can affix a 3.5” adapter bracket to your 2.5 inch SSD. These brackets are included with the Desktop Installation Kit.
If you do not have a 3.5” bracket, you can purchase it online or at most local IT stores.