My Apple Watch doesn’t is not connecting to my iPhone

There’s usually a means to perform that watchOS update, either on that same display on the paired iPhone, or directly on the Watch itself.

Click or tap > Update your Apple Watch - Apple Support

What have you already tried, what happens when you tried it, and which Apple Watch, and which watchOS version?

is this a new Apple Watch, or a new-to-you Watch that might (for instance) be Activation Locked to another Apple ID?