Helices position

Now that you are flying you will certainly have some crashes (its normal, don't be affraid).

If in one of those accidents you lose or break one of the helices you just have to use one of the substitution helices that come with the drone (almost all drones have a spare of four helices to change if needed).

Be aware that each helice belong to a motor.

That said you just have to look to each one and use the correct one, if you change the wrong helice your drone will not fly. Here is a schematic of the right helice position on a four motor drone.

As you see the top left helice is the same as the lower right one, so there is no problem if you break/lose the same helice twice.