I cant update my apple watch?

If you have a first generation iPhone SE, the highest it can update to is iOS 15.7.x. It cannot be updated to iOS 16. And if you want your watch to update to WatchOS 9, you won't be able to do so, as you must own an iPhone which can run iOS 16, which a first generation Apple Watch SE is not capable of.

On your iPhone open the Settings App. Tap General > About > Model and Version and report back. It will be a while before I can reply as I'm leaving work and won't be back online for a while.

What do you want to update your WatchOS to? And what exact model iPhone do you own?

Your tag line says you own Apple Watch SE on WatchOS 8. If you want to update to WatchOS 9 (current version is WatchOS 9.3), it requires an iPhone which is updated to iOS 16 (current is iOS 16.3). That means you need to own an iPhone 8 or newer or an iPhone SE 2 or newer.

I'm glad I could help. And yes, if you want to update your watch to WatchOS 9, which it is very capable of using, you will need to buy a newer, more modern iPhone. Your phone will get no more updates and it won't be long before apps and some features no longer run on the outdated iOS on your iPhone.