Hello

I have just bought a ticket to Peru with Avianca using my miles, i am having difficulties to assign the seats, when i checked the status of my booking on united.com i saw that the system radomely assigned the seats but i wanted to change those seats because i do not like them, i tried to do it in the website of Avianca but was not useful nor united.com

Could somebody tell me what i can do, will i have to wait until the check-in to assign another seat?