Volfan4life87 is right. Those are the only two ways I know of. In the past I have selected a high performing yeast, like Lalvin 1118, and let the yeast ferment my mead to completion. If you were to drink it at that point it would be very dry, and I like my mead a bit sweater. So, I add potassium sorbate, which kills all the yeast in the wine. Now that the yeast is dead you can add more honey or sugar to the mead and it will no longer ferment. You can get potassium sorbate at any homebrew shop or online and the instruction on how to use it come with it. This option will work for any alcohol percentage mead.
The next option is to select a yeast with a certain alcohol tolerance that you are looking for in your mead lets say 12%. Then add enough honey to your water to get the potential alcohol percentage to 13%, 14%, or 15%. Depending on how sweet you like it. I recommend going two percentage points above the alcohol tolerance of your yeast. An important thing to note using this method is that just because a yeast is rated at say 12% alcohol tolerance, that doesn't always mean that it will stop fermenting at 12%. If the conditions are just right for the yeast (temperature, water pH, yeast nutrients, etc.) the yeast can go 1 or 2% higher that it is supposed to. This is from experience. This method usually works but not always. If the yeast does go higher your can just continue with option 1 using potassium sorbate and back sweatening.
This next option is something that I am trying right now. You want to get your potential alcohol to 8 to 10%. You could try 12% but I wouldnt go much higher. Do everything like your normally would when making your mead, except instead of using wine yeast, or mead yeast, use an ale yeast. Maybe Safale US-05, or any ale yeast. When you do this, generally the ale yeast will leave behind some residual sweetness (in theory). I know that quite a few people do this, but I personally will have to wait until mine is done. You can also find sweet mead yeast out there that do essentially the same thing only a little better and they can tolerate more alcohol. This option will be ready to enjoy sooner than the higher alcohol options.
If your goal is higher alcohol I would say ferment out and back sweeten, or find a sweet mead yeast and try it out. I believe white labs has one called 720 Sweat Mead, and it should tolerate up to 15% alcohol.
I hope I helped, kind of rambled on. I had the same question earlier in my brewing adventures. There are quite a few good youtube videos on back sweetening.