You can use the following syntax to change the order of the items in a ggplot2 legend:

scale_fill_discrete(breaks=c('item4', 'item2', 'item1', 'item3', ...)

The following example shows how to use this syntax in practice.

Example: Change Order of Items in ggplot2 Legend

Suppose we create the following plot in ggplot2 that displays multiple boxplots in one plot:

library(ggplot2)

#create data frame
df <- data.frame(team=c('A', 'A', 'A', 'A', 'B', 'B', 'B', 'B', 'C', 'C', 'C', 'C'),
                 points=c(6, 8, 13, 16, 10, 14, 19, 22, 14, 18, 24, 26))

#create multiple boxplots to visualize points scored by team
ggplot(data=df, aes(x=team, y=points, fill=team)) +
  geom_boxplot()

To change the order of the items in the legend, we can use the scale_fill_discrete() function as follows:

library(ggplot2)

#create data frame
df <- data.frame(team=c('A', 'A', 'A', 'A', 'B', 'B', 'B', 'B', 'C', 'C', 'C', 'C'),
                 points=c(6, 8, 13, 16, 10, 14, 19, 22, 14, 18, 24, 26))

#create multiple boxplots to visualize points scored by team
ggplot(data=df, aes(x=team, y=points, fill=team)) +
  geom_boxplot() +
  scale_fill_discrete(breaks=c('B', 'C', 'A'))

Notice that the order of the items changed from: A, B, C to B, C, A.

We can also use the labels argument to change the specific labels used for the items in the legend:

library(ggplot2)

#create data frame
df <- data.frame(team=c('A', 'A', 'A', 'A', 'B', 'B', 'B', 'B', 'C', 'C', 'C', 'C'),
                 points=c(6, 8, 13, 16, 10, 14, 19, 22, 14, 18, 24, 26))

#create multiple boxplots to visualize points scored by team
ggplot(data=df, aes(x=team, y=points, fill=team)) +
  geom_boxplot() +
  scale_fill_discrete(breaks=c('B', 'C', 'A'),
                      labels=c('B Team', 'C Team', 'A Team'))

Notice that the legend labels have changed.

Additional Resources

The following tutorials explain how to perform other common operations in ggplot2:

How to Remove a Legend in ggplot2
How to Change Legend Position in ggplot2
How to Change Legend Size in ggplot2
How to Change the Legend Title in ggplot2