group_by(mtcars, cyl) %>%
summarise(mean(disp), mean(hp)) %>%
`colnames<-`(c("cyl", "disp_mean", "hp_mean"))
# or
# `names<-`(c("cyl", "disp_mean", "hp_mean"))
# setNames(., c("cyl", "disp_mean", "hp_mean"))
# cyl disp_mean hp_mean
# 1 4 105.1364 82.63636
# 2 6 183.3143 122.28571
# 3 8 353.1000 209.21429