Recently replaced battery in sons 91 civic ex, having problems getting it to idle properly, should ecu learn this on it's own or does it need to be taught? And if so how ?

The idle issue is not likely a function of the battery being changed. You may want to begin by cleaning the Idle Air Control Valve (IACV), throttle plate, and throttle body:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-7.html#post1054149

If the idle rpm is just low, simply loosen the inside 12 mm jam nut on the throttle control and tighten the outside 12 mm nut to increase the rpm.

If the tachometer is fluctuating up and down, the Ignition Control Module may be failing, which is common on older Hondas:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post260436

Here's how to replace it:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-4.html#post265896

I would also replace the coil, since you will be deep into the distributor. I recommend replacing these problematic components every 120,000 miles orr 10 years, whichever comes first.

This link may also be of interest:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-7.html#post842969