If your Apple Watch battery suddenly seems worse after installing a watchOS update, the update itself may be the reason, but not necessarily because anything has gone wrong.
Apple says some update-related tasks continue in the background even after the watch is ready to use again. Those jobs can temporarily increase power use, which is why battery life can look noticeably worse for the first few days.
That makes it worth waiting before changing a long list of settings. If the watch was fine before the update and the battery drop appeared immediately afterwards, give it a little time to settle.
Keeping the watch near its paired iPhone can also help. Apple Watch is most power efficient when it can use Bluetooth to communicate with the iPhone, rather than relying more heavily on Wi-Fi or cellular.
If the problem continues after several days, check Battery Health on the watch under Settings > Battery > Battery Health. Maximum Capacity gives you an indication of how much charge the battery can still hold compared with when it was new.
Low Power Mode can help on unusually heavy days, but it should not be necessary to leave it enabled permanently just to compensate for a recent update.
A short-lived battery hit after watchOS updates is common enough that it is usually best to monitor the watch for a few days before assuming the battery itself has deteriorated.