generaly speaking, the Z170 Plattform supports 7th Gen CPUs. but the BIOS needs to be adjusted by Dell / Alienware and i don't know if they did it or not. you will have to Update your BIOS to the latest Version before you swap the CPUs, that's for sure. i think Alienware Eimy or Alienware Rodrigo can tell you more about this.

I think this thread covers it:

If you must upgrade CPU, I suggest you stick the Intel CPUs that the machine originally shipped with during its run. Alienware support can probably give you that list if you can't find it already posted here somewhere.

Intel® Core™ i3-6100 Processor (3M Cache, 3.70 GHz) Product Specifications 

• # of Cores 2
• # of Threads 4
• Processor Base Frequency 3.70 GHz
• Cache 3 MB SmartCache
• Bus Speed 8 GT/s DMI3
• TDP 51 W
• Memory Types DDR4-1866/2133
• Sockets Supported FCLGA1151
• Lithography 14 nm

Intel® Core™ i5-7400 Processor (6M Cache, up to 3.50 GHz) Product Specifications 

• # of Cores 4
• # of Threads 4
• Processor Base Frequency 3.00 GHz
• Max Turbo Frequency 3.50 GHz
• Cache 6 MB SmartCache
• Bus Speed 8 GT/s DMI3
• TDP 65 W
• Memory Types DDR4-2133/2400
• Sockets Supported FCLGA1151

• Lithography 14 nm

Bios may need to be updated but the other spec's are Aligned.  Ram would need to be replaced because what you have is TOO SLOW.