how to use update() for random part in lmer()?

I would like to use the update() function to update the random part of my model, specifically, adding a random effect. Most examples (help("update"), help("update.formula"), lme4:mixed effects modeling with R) focus on the fixed part of the model. How would I go from fm0 to fm1 using update() in the example below?

library(lme4)
(fm0 <- lmer(Reaction ~ Days + (1 | Subject), sleepstudy))
(fm1 <- lmer(Reaction ~ Days + (1 + Days | Subject), sleepstudy))