Actualización

It's technically possible to pair a watch 1 or 2 to a newer phone,but in reality this is proving almost impossible for just about all the 1&2 owners on this forum.

Newer phone's require the watch to have updated hardware to cope with ios15 and unfortunately because of the age of these watches the hardware is very limited to what it can do.

Then there is the subject of storage,these watches have very little storage space and the average software update now requires gigabytes of space rather than megabytes and again these watches are very limited with the amount of storage space they have.

It is recommended that even a watch 3 should be unpaired and repaired in order to free up enough space to update.

What model of phone is it and what version of ios is it running?

What model of watch is it and what version of WatchOS is it running?.

What troubleshooting steps have you tried?.

What errror messages are you receiving?.

Has this watch ever been paired to another phone?(Very important).