Generally it states on the "Replace with..." line. If you have any specific questions it would help a little.
But generally: Replace with - can only be upgraded once unless the unit comes with 2 or more of that specific weapon as a base. Upgrade with one - can only be upgraded once Upgrade any unit with one - also only one per squad. Upgrade all models with - here you can select as amny upgrades as you wish. Some times it comes as "Upgrade all models with one" and in this case there is only one upgrade.
If a model is armed with for instance a Assault riffle and CCW they can be upgraded at the "Replace one Assault Rifle and CCW:" with a pistol and CCW weapon. you can then check the list for upgrading the pistol separately and the CCW separately. Technically you have upgraded the model three times from different lists which is completely valid.