How to replace values in differents columns with values in another column? (R)

Question

library(tidyverse)

reprex for you to reproduce:

library(tidyverse)

tibble(
  x1 = c(1, 2, NA, NA, 5),
  y1 = c(4, 3, NA, NA, 7),
  x2 = c(NA, NA, 6, 7, NA