why can i update the watch

Ensure your paired iPhone is updated to current.

Here is how to update Apple Watch:

… Update your Apple Watch - Apple Support

If you have issues with Series 3, then un-pairing and re-pairing can provide a means to update:

… If you don’t have enough space to update your Apple Watch - Apple Support

The apple watch 3 GPS only has only 8GB of internal storage, most of that is taken

up by the operating system and other critical software with not much space left for the

update.So installing or even pairing to an ios15 phone is proving nearly

impossible for just about all the watch 3 users on this forum.

same applies to Apple watch 1 and 2 they are almost impossible to either pair or

update with an ios 14 or ios15 phone.

Your best course of action is to call in at an apple store and ask them to pair/update it for you.