Hi
how can I edit a now weapon for my nation ?

Depends what you want to do:

1 - change the national weapon to one of the other existing national weapons

2 - keep the existing national weapon stats but change the graphics

3 - introduce a new national weapon with unique stats (and a new graphic)

Option 1 is fairly straightforward - Croatia did that recently. You just need a reason for the change.

Option 2 relies on you finding sample graphics and convincing Hoern and the graphics admin to redraw this.

Option 3 needs to start with a proposal to the Tech-Mods, explaining the concept of the weapon, why it's more appropriate than the current etc.

if you want to have the option to buy it you should build a forge in one of your provinces (all players choose castle for that ) After forge is build you can buy your national weapon from market in that province.
If you don`t like your national weapon but like another nation weapon you must go to the province where that nation have a forge, ask permission to enter in that province (otherwise you risk to be imprisoned) and buy that weapon you like