How display length of branches in phylogenetic tree

You can do it by extracting edge lengths and using edgelabels().

# Load package
library(ape)

# Create data
t <- read.tree(text="(F:4,((D:2,E:2):1,(C:2,(B:1,A:1):1):1):1);")
plot(t)
edgelabels(t$edge.length, bg="black", col="white", font=2)