How to replace pattern on whole column of Dataframe in r?

Question

i Have a data frame

df= data.frame(nam=c("abc.R","bcf.R","hhhR.R","vf.R")
               val=c(NA,2,3,4))

I want to replace character "." with "-" in df$nam

Required Output

df= data.frame(nam=c("abc-R","bcf-R","hhhR-R","vf-R")
               val=c(NA,2,3,4))