I have Ubuntu installed. I created 2 more partitions for Linux Arch and failed to install it due to choosing the wrong packages via package manager. It will work on my computer for sure (it already did). So I'm going to re-install it.The reason I created 2 partitions (/boot and main /) for Arch is that I'm on UEFI system and it's x64 also. That's a must.So do I have to remove all the files from these 2 partitions before re-installing Arch or can I just re-install it without it? If case I have to remove them, how would I do that? Just mount them (partitions) while I'm on Ubuntu and remove via rm -r all the files in them? 
How do I properly re-install an OS to the same partitions after failing for the first time?
boot;dual boot;grub;64bit
You should probably wipe and reformat those partitions.Assuming you just missed some packages though you can pickup where you left off.Boot the Arch livedisk mount the partitions and use archchroot to install the correct ones.  This is also a common strategy when you need to recover from driver or kernel problems.