You must need a jailbroken iPhone if you don’t have one visit jailbreakme.com and jailbreak yours.
1. Launch Cydia.
2. Add new source by Taping on the Manage Tab and then on Sources.
3. Tap Edit and then add, type the following URL: http://repo.benm.at/

4. Tap done and then Tap this repo and Tap on Frash to install it.
5. Reboot your iPhone.

Or you can also install from deb file

Step 1: First up, you will need to jailbreak your iOS device.

Step 2: Next, you will need to install OpenSSH. To do this, Open Cydia, touch on “Search” tab and then search for “OpenSSH”. Install this app and reboot your iPhone.

Step 3: Connect your iPhone with your computer. Make sure iTunes is not running.

Step 4: Download and install WinSCP for Windows. Enter the following details to login to your iPhone:

WinSCP

Hostname: The IP address of your iPhone/iPad/iPod touch. Setting –> WiFi –> <Your Network Name>
User name: root
Password: alpine
Protocol: SCP

Download Frash-0.02.deb file from the source link given below.

Step 6: Navigate to /var/root/Media directory and create a folder named “Cydia”. Inside this Cydia folder, create another folder and name it “AutoInstall”.

Step 7: Copy over the Frash-0.02.deb file in this “AutoInstall” folder.

Step 8: Restart your iPhone and you are done. Now simply browse any Flash based site, click on the “Flash” text to play the Flash content.

Download Frash-deb or Mirror
Source: Iphone News