If you do not have an area of your admin back end to adjust the weight of your product you will need to either add that functionality to your back end area or make the change manually in the database.
If you used DataAssist to make the backend you should be able to edit the update page to put in an input for the weight so you can change it. Let us know if you would like to go this route and what questions you have about this.
If you are not referring to your admin back end please let us know.
If you are getting the shipping back through the server behavior it will be based on the item weight, package size, the origin and destination locations along with the service type. Do you have a specific example of where the shipping values that are being returned are not correct? If you need to you can add in an increment price to increase the shipping that you charge to cover your costs.