There is a lot of great science fiction that deals with this (and some not-so-great). There are a lot of levels to the problem: at a minimal level of evolution we’d expect to see single-celled life. And then if it’s been evolving for billions of years multicellular life on up to possibly intelligent life. At the lowest levels, how will these microbes interact with our own biology? Could they infect and sicken us? Is the alien world’s biology compatible enough with our own (carbon based, DNA based) that we might be able to actually grow Earth plants and animals there — or even eat alien life? And if there are actual sentient and/or intelligent species already on the alien world, how will we interact? Will it be war from the start? Will we be able to communicate? Will we ultimately be able to share the planet?
If it is a matter of humanity’s survival, I imagine our settlers will be quite desperate and do whatever is needed to remain on the new world. If we know of other habitable worlds nearby that are more primitive, then our settlers may have the option to move on and leave the inhabited alien world to its own intelligent species. It comes down to a timeless philosophical question: how far do we go to preserve our own species? It may not be morally right to wipe out alien life to make way for our own, but if we have no other choice I am sure that’s what our settlers will do.
Ultimately the entire universe has its own “food chain” with apex predators. Humanity is Earth’s apex predator, but there are likely other advanced species out there that eclipse us on an interstellar scale.