how to replace nth character of a string in a column in r

My input is

a<-c("aa_bbb_cc_ddd","ee_fff_gg_hhh")
b<-c("a","b")
df<-data.frame(cbind(a,b))

I want my output to be

a<-c("aa_bbb-cc_ddd","ee_fff-gg_hhh")
b<-c("a","b")
df<-data.frame(cbind(a,b))

please help