You may upgrade your weapon in the equipment tab of the Shop window. If an upgrade is possible, a weapon icon will be marked with an arrow sign . If it is not marked, it means you can't upgrade this weapon or it has been upgraded to the maximum level. Weapon upgrade for silver coins or canned food can fail.
When you open the weapon upgrade menu, you will see three boxes with different options:
Each of these options allows you upgrade the weapon, but the first two types (an upgrade for silver and cans) may end up in failure, as the success rate is 30 and 70% per try respectively.
Regards, Nexters Support Team