Red triangle after replacing 12 volt battery

You need the codes read. You can try disconnecting the 12 volt for a few minutes and then reconnect and see if that does the trick it's possible. Is your check engine light on red triangle is on then generally you have stored codes You need to get those so you know what you're looking at basically

 

try holding the start button down for a few seconds, or press it three times.

if no luck, measure the voltage at the jump point under the hood

 

Which of those you get is important. One that won't show all the codes you have is in some ways worse than not having one (because one day it will make you post something like "I have these warning lights on and no codes" and then we'll have to say "dude, you need a better code reader").

Here's a thread that reviews a bunch of good and better options. If the one you're getting is one of the better ones there, then no worries.

Gen2 OBD2 app review | PriusChat

 

Do you KNOW that the new battery is fully charged ?
Did you check the OTHER ends of the main cables during the replacement ??
Sometimes there are some small wires connected to the positive post too; if so, are all of them reconnected ?