I cannot update my series 3 watch

To update your watch, open the watch app on your iPhone. Select General > Software Update. Your Apple Watch will check to see if it is eligible for an update. If it is, you will be prompted to update. If you do not see the prompt to update then it means your watch’s OS It is as high as it will go. If this does not answer your question, then please give us more detail regarding what you have tried and what error message you may be seeing.