Define some data:

library(dplyr)

## 
## Attaching package: 'dplyr'

## The following objects are masked from 'package:stats':
## 
##     filter, lag

## The following objects are masked from 'package:base':
## 
##     intersect, setdiff, setequal, union

library(ggplot2)

df = tibble(
  x=c(0, 0, 1, 1),
  y=c(0, 0.1, 1, 1.1),
  group=c("a", "b", "a", "b"),
  ymin=c(-0.5, -1, 0.5, 0),
  ymax=c(0.5, 1, 1.5, 2)
)

If we “just” plot it:

ggplot(df, aes(x, y, ymin=ymin, ymax=ymax, fill=group)) +
  geom_ribbon()

Reordering the data frame doesn’t change the plotting order:

df[rev(1:nrow(df)),] %>%
  ggplot(aes(x, y, ymin=ymin, ymax=ymax, fill=group)) +
    geom_ribbon()

But changing the factor order of the grouping variable does:

df %>%
  mutate(group=factor(group, c("b", "a"))) %>%
  ggplot(aes(x, y, ymin=ymin, ymax=ymax, fill=group)) +
    geom_ribbon()