There's a very simple cheat to set whatever high score you want in Flappy Bird. Here's the steps. All you need is a fresh install of Flappy Bird with a high score above 1 in it and root access on your device. Oh, and ES File Manager is required too.

Just install the fresh version of Flappy Bird (don't worry about losing your high score because you're about to set your own!).
Play it once just to get a high score above 1 in the score sheet.
Go to ES FIle Manager and make sure you have Root Explorer turned on in the navigation panel, accessed via the world/phone icon in the top left (or swiping left to right from that area).


In the Local pane, you want to go to the Device root directory and then Data and then Data again.
Now just look for the com.dotgears.flappybird folder (you can search for flappybird if necessary) and open it.


You'll see a couple of folders, open the Shared_prefs folder.
Open FlappyBird.xml and choose ES Note Editor when the popup window appears.
Open the action overflow menu (top right, three dots) and hit Edit.


Change the score to whatever you want it to be. I went for something at least mildly believable: 237. If you enter 999 I think your friends will know something is up.
Save your changes and play again. You'll now see your new high score on the scoreboard.

PS: I don't take any responsibility if you mess up.