This is a generalized way in which you do not have to remember the exact location of the variable:
# df = dataframe
# old.var.name = The name you don't like anymore
# new.var.name = The name you want to get
names(df)[names(df) == 'old.var.name'] <- 'new.var.name'
This code pretty much does the following:
names(df)
looks into all the names in thedf
[names(df) == old.var.name]
extracts the variable name you want to check<- 'new.var.name'
assigns the new variable name.