W12x65 is a standardized profile and will appear in your Tekla Structures catalog. You need to select your element (the column) and open the Properties tab. In the Attributes tab you will have a Select button next to the Profile description. Press on it. If your profile doesn't appear by default, you need to check Show all profiles from downside. Select your profile by double click and press OK. You can modify the profile dimensions on the General tab by clicking the Value box and replacing the existing value with a new one.