Cannot upgrade to watchOS 5.3.1 using Apple Watch series 4

Hi, i have got an iPhone 6 with iOS 12.4.2 and an Apple Watch series 4 with watchOS 5.2.1 installed. I don't know why i can't upgrade to the latest watchOS firmware compatibile with my phone that is watchOS 5.3.1 which is focused about resolving security issues. How can i resolve it? Thank you