Replace a value in a data frame based on a conditional (`if`) statement

another useful way to replace values

library(plyr)
junk$nm <- revalue(junk$nm, c("B"="b"))

Short answer is:

junk$nm[junk$nm %in% "B"] <- "b"

Take a look at Index vectors in R Introduction (if you don't read it yet).

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EDIT. As noticed in comments this solution works for character vectors so fail on your data.

For factor best way is to change level:

levels(junk$nm)[levels(junk$nm)=="B"] <- "b"

Easier to convert nm to characters and then make the change:

junk$nm <- as.character(junk$nm)
junk$nm[junk$nm == "B"] <- "b"

EDIT: And if indeed you need to maintain nm as factors, add this in the end:

junk$nm <- as.factor(junk$nm)

As the data you show are factors, it complicates things a little bit. @diliop's Answer approaches the problem by converting to nm to a character variable. To get back to the original factors a further step is required.

An alternative is to manipulate the levels of the factor in place.

> lev <- with(junk, levels(nm))
> lev[lev == "B"] <- "b"
> junk2 <- within(junk, levels(nm) <- lev)
> junk2
   nm val
1   A   a
2   b   b
3   C   c
4   D   d
5   A   e
6   b   f
7   C   g
8   D   h
9   A   i
10  b   j
11  C   k
12  D   l

That is quite simple and I often forget that there is a replacement function for levels().

Edit: As noted by @Seth in the comments, this can be done in a one-liner, without loss of clarity:

within(junk, levels(nm)[levels(nm) == "B"] <- "b")