Here in the original manuscript, Oliver Cowdery initially wrote preach rather than teach. But he immediately corrected the word by erasing the pre and overwriting it with te. Either reading is theoretically possible here. In Alma 8:4, scribe 2 of 𝓟 made the same error when he copied from 𝓞 into 𝓟, writing preach initially and then immediately correcting it to teach (in this case by crossing out the preach and then writing teach inline).
Summary: Follow in Alma 38:15 the immediately corrected reading in 𝓞, namely, teach rather than preach (“now go my son and teach the word unto this people”).