You can use the stringr library:
library('stringr')
a <- runif(10)
b <- letters[1:10]
c <- c(rep('A-B', 4), rep('A_B', 6))
data <- data.frame(a, b, c)
data
# a b c
# 1 0.19426707 a A-B
# 2 0.12902673 b A-B
# 3 0.78324955 c A-B
# 4 0.06469028 d A-B
# 5 0.34752264 e A_C
# 6 0.55313288 f A_C
# 7 0.31264280 g A_C
# 8 0.33759921 h A_C
# 9 0.72322599 i A_C
# 10 0.25223075 j A_C
data$c <- str_replace_all(data$c, '_', '-')
data
# a b c
# 1 0.19426707 a A-B
# 2 0.12902673 b A-B
# 3 0.78324955 c A-B
# 4 0.06469028 d A-B
# 5 0.34752264 e A-C
# 6 0.55313288 f A-C
# 7 0.31264280 g A-C
# 8 0.33759921 h A-C
# 9 0.72322599 i A-C
# 10 0.25223075 j A-C
Note that this does change factored variables into character.