Alexa gets its weather from Accuweather, and some users prefer other sources. Unfortunately, there isn't a way to change this default weather source. But, you can add an Alexa Skill to your Echo device. All you need to do is pull up the Alexa app on your smartphone and tap on the More icon in the lower-left corner. From here, you can tap on Skills & Games to search for the weather source you prefer to use. After you've added the skill, you can simply say, "Alexa, ask [weather skill] what the weather is" or something to that effect.