After the additional clarification in the comments upgrading the CPU should be reasonably straightforward. But if you don't have any experience with this it is a good idea to have someone with experience around.
This is rather expensive stuff to make a mistake on...
As always with a CPU upgrade start by making sure the computer has the latest UEFI firmware. Upgrade first if it is an older version.
Then remove all external cables (especially the power cord) and open the case.
If you have ESD measures use them. If you don't have them at least touch a grounded object (like a nearby radiator or the metal housing of a grounded electrical appliance), before you start and every few minutes while you are working on this.
Remove the cooling fan/heatsink assembly from the CPU. This should be some kind of mechanical fixing. Usually some screws or a retaining clip. That can vary.
Undo those first and then pull the cooler off straight upwards. You don't want to pry it off or put uneven pressure on the underlying CPU.
The cooler may be really stuck because the cooling paste (or pad) between CPU usually acts as glue. A slight rotating motion may help.
(P.s in order to give yourself some working room you may want to temporarily remove the video-card and other PCIe cards if present.)
Then follow the instructions from the video you already linked in the comments. It is an excellent instruction. Where they are removing the protective plastic cover from the frame you will be removing the old CPU instead, but it works exactly the same way.
After you have installed the CPU as per the video the last step is to re-instate the cooler assembly.
Before you do that you will have to remove the old cooling-paste (or pad) from the bottom of the heatsink and try to avoid scratching the surface of the heatsink while you do that. You want this as smooth as possible.
First scrape of the majority with a soft plastic spatula (old credit card works pretty well too) and then wipe of the rest using a tissue and some iso-propyl alcohol.
Then apply a couple of drops of fresh cooling paste on the bottom of the heatsink. (Don't overdo it. You only need the paste to form a thin film between CPU and heatsink. It shouldn't squeeze out from underneath the heatsink.)
Then place the cooler back on the CPU and fasten it, in the reverse order of taken it out.
(The Dell website may have a instruction on replacing the cooler assembly. Look at that first!)