Welcome
Yes, there is a free way. If you received 2 DVD's with your purchase, just remove the OS from one machine and then put the 64 on the same or another machine.
If you do not have the 64 bit DVD a call to Micorosoft telling them that you have a key number, but want to switch to 64 bit, it is very possible that they may send you a DVD for cost.
You are only prohibited from using 32 bit and 64 bit, at the same time on two machines, if you only have one key number.