Tim = i.e. "akasa TIM Clean, CPU and Heatsink Cleaner, removes thermal pads and silver based thermal grease."
I suggest you contact them to deal with this. You paid for a pre-built system, and this certainly isn't right. Moreover, if you don't know what you're doing, then the heatsink on a CPU is the last thing you want to be playing with.
However, if you really think you can do it, then do a search on youtube, there are plenty video guides on how to afix heatsinks, search for the one you have. It's perhaps easiest to see it done by someone else.
Also, I personally would clean the thermal grease off using TIM and reapply some new stuff.
I suggest you contact them to deal with this. You paid for a pre-built system, and this certainly isn't right. Moreover, if you don't know what you're doing, then the heatsink on a CPU is the last thing you want to be playing with.
However, if you really think you can do it, then do a search on youtube, there are plenty video guides on how to afix heatsinks, search for the one you have. It's perhaps easiest to see it done by someone else.
Also, I personally would clean the thermal grease off using TIM and reapply some new stuff.