It's not difficult, Uni, although I believe your Seagull has hydraulic brakes. If that's true, did they send the whole lever and caliper.
The problem might be that I hear bike shops are booked up with repair work and you might have to wait for a drop off appointment and come back a couple of days later to pick it up. And with business that good, who needs to puzzle out an ebike.
It's not hard to take off the fake leather grip. I squirt some WD40 under the rubber and with some work, they twist off. I put them back on with a little soapy water. When the water dries, they're tight again.
After the grip is off, one allen head screw holds the brake lever. Ecotric likes to wrap everything in spiral wrap, so you have to undo that to get the hydraulic cable and the brake wire free. The brake wire will plug into the controller so someone had to unplug the old lever and reattach the new one. The spiral wrap can be re-used. The only non-bike work is tracing the wire to where it goes on your bike, and than changing it at the connector.
SInce it's hydraulic, replacing the caliper on the rear frame is pretty easy, and so is adjustment. Look it up on youtube.
If this is too hard, then call around your local shops or look for a mobile repair guy. Join the Ecotric groups on facebook. Probably a few NJ members. A lot more Ecotric owners there than here.
I have a Ecotric 20" fat tire bike with mechanical disk brakes, but I've put hydraulic brakes on another bike. That one started out with rim brakes.
