This question deserves an honest answer, not a deflection. If you've been told to just trust without thinking, the Church does not require that of you.
The context
The Canaanite cultures named in Scripture practiced child sacrifice (Lev. 18:21; Deut. 12:31 — corroborated by extra-biblical evidence at Carthage and elsewhere) and ritual prostitution. The conquest is framed as judgment on specific practices, not ethnic cleansing — the same judgment Israel is told it will face if it does the same things (Lev. 18:28; and which Israel did in fact face in the exile).
The rhetoric
Scholars like Paul Copan and Richard Hess have shown that ANE conquest accounts routinely use total-destruction rhetoric ("utterly destroyed all that breathed") even when the actual battle was limited. Joshua 11 says the cities were utterly destroyed; Judges 1 — describing the same period — says Canaanites kept living among Israel. The rhetoric is formulaic; the reality was more limited.
What the conquest is not
It is not a model for Christian action. The New Testament explicitly redefines the battle as spiritual (Eph. 6:12). The Church Fathers — Origen, Gregory of Nyssa, Maximus — read these texts typologically: the Canaanites are the passions to be put to death within us, not enemies to be killed without.
The Orthodox lens
The God who said "utterly destroy" through Joshua is the same God who in Christ said "love your enemies" — and then proved it on a cross. The trajectory of Scripture is toward the Lamb, not the sword.
A pastoral word
You can hold this question open with reverence. Real Christians have wrestled with it for centuries. The answer is not to soften God or to dismiss the text, but to read it through the One who bled rather than retaliate.