Often, leagues have different rules around how playoff matchups are determined.
Sleeper fantasy football leagues allow commissioners to manually modify playoff seedings so you can do things like give the highest scorer the second place seed and a bye week.
This feature is available the week leading up to the postseason on the playoff bracket interface for commissioners.
If you have different rules for who makes it into playoffs or the seeding order, go to league settings on your phone. Then, scroll to the bottom, and tap on "edit playoff seeding".
Next, tap on the seed that you want to update and you'll see a list of teams show up at the bottom of your phone.
Tap on the team that you want to switch.
IMPORTANT: after you've made all the changes, tap update in the upper right before exiting the screen.