first of all you have to land and be at the equipment dealer. if you dont want to sell a weapon that is already attached, then just press the icon in the top left of the weapon picture, that unataches it. when you scroll down to the bottom where the unatached weapons are, click on the top left of the weaopn you want to attach.

you can't really choose which spot you want the weapon in, they just go to the first available one that they fit.

hope i didn't mess this explanation up too much, eh

I'd rather be sucked off by a mountain goat with peanut butter on my penis - Centy

You have no idea how long it took me to figure out the weapon switching I used to sell the mounted and the one I wanted to mound, then re-purchase the one I wanted to mound... wasted a lot of money that way...

how do you know which weapon goes where? i got 2 weapons of the same both on the same wing, feels werid when firing

Learn your ship, I fire one weapon at the time from the list to see where the mount is, Yeah it took me some time as well, but once I got it, well I started installing Drails and Xeno Scorpions...

i think you will find that the gun mounts on all fighters go in this order

MONUTS 1 & 2: Outer most (usually wing tips)
MOUNTS 3 & 4: Middle points (mid wing)
MOUNTS 5 & 6: Inner most (cockpit sides)

at least that is how they are done for the Titan. of course an odd number of mounts usually has the last mount along the centerline of the ship.

hope that helps. would appreciate anyone telling me if i am wrong but i think is applies to all types of fighters (no idea about freighters i never fly them)