You can't judge a device with its first charge, if you just got it today give it a day or two and make sure to fully charge it. I'm getting pretty great battery life from my Essential especially standby time whivh blows away my Note 8 standby time. Screen on time I'm getting from 5 - 6 hours screen on time over my day and if I use it like the average person does I can get a day and a half with around 4 1/2 hours SOT.
Charge it overnight and give it another shot or charge it full and then perform a factory reset while plugged in.
I am however running Android 8.1 Oreo beta so you will also get better battery when you update to Oreo 8.1.
If you really want to change it you have 14 days to exchange or return it, visit to store you bought it at or if you purchased it online you'll need to contact the webstore and request a return envelope.