In this case, the level order could be set in the first line; however, if there are many levels (and you don't want to type out all of the levels explicitly), the above method is preferred. Again, if there is a better way to do this, please let me know in the comments.
## generate datax = factor(sample(letters[1:5],100, replace=TRUE))
print(levels(x)) ## This will show the levels of x are "Levels: a b c d e"
## To reorder the levels:
## note, if x is not a factor use levels(factor(x))
x = factor(x,levels(x)[c(4,5,1:3)])
print(levels(x)) ## Now "Levels: d e a b c"