To switch which phone number is your assigned primary (default) number, click your name in the top right corner and select "Numbers"
Next, check out the stars to the left of each number on your account. The number with the filled in star next to it is your Primary number.
Simply click the (blank) star next to the number you want to switch to be your primary number and you will see the star fill in purple next to that number. You can change this primary number anytime using these same steps.
If there is no star icon next to the number, that means it's not a number you own or it cannot be used as a primary number.
A number with this icon next to it means it is being used as a secondary number. Secondary numbers cannot be used for anything other than sending group messages. See this article HERE for more details.