How do I reset ecu after long disconnection of it's battery?

i would suggest dealer only ,a lot of the software is dealer only encrypted ,you could try disconnecting the battery and leaving it for a hour then reconnecting battery and see what happens

The only reset I know of on OBD-1 systems (like yours) is to disconnect battery for a couple of minutes. Take off the negative cable for a few minutes.

You need a scanner to reset the system.

You can reset the ECU by removing the car battery......leave the battery disconnected for one hour .....keep the brake pedal pressed for 5 min to discharge any residual electricity.

the ECU , reset on all cars has 3+ ways.
all do.
keyon
1: scan tool push reset button (best of best)
key off.
2: pull the ECU memory fuse for 5min put back,
3: pull battery neg lug for 5min put back.
4: Pull ECU, (gross way) but does 2 trick.

usa answers only... and not stated by you. (yours)
that fuse is called

ECM 30amp, F-3 (fuse relay box)
it is the memory fuse, and is hot all the time, (for memory retention)

ecu=pcm=ecm


proof.

typically, to reset an ecu, you can disconnect the battery for a while (maybe an hour or two to be sure). Re-connect the battery and the ecu will have to re-learn again.

Well it's probably just a coincidence but, either buy a code reader/scanner and reset the ecu to ready or go to autozone or advanced auto and have them read the codes and reset the ecu. Some of these auto stores will not erase the codes. or disconnect the battery from the car and let it sit about 30 minutes. That should reset the ECU. Drive the car approximately 50 miles if you disconnect the battery because it will need to relearn your emissions again. If the check engine light comes back on then you either have a bad O2 sensor or your cat is bad, but I'd lay money on your O2 sensor.

ecu can be reset by disconnecting battery. this is likely not your problem..

Do an ECU drain first. ie disconnect battery for 30 secs. foot the brake and reattached battery thereafter. If battery check light appeared again, disconnect battery while engine is on. If engine dies, you have an alternator problem - replace it. If engine remains on, you have a voltage regulator problem.
If battery check light disappeared, ECU drain resets sensors thus fixed your problem.

Try disconnecting and connecting back battery, it reset the ECU, it can be a problem with the computer, but also a sensor that had gone faulty when replacing battery.
It can also be a short or some open contact caused by moving some wire or connector when the battery has been replaced.
If you cannot reset the ECU by disconnecting battery for a while go to a garage to check what the fault is.