One of the best ways to play FLV files on a Windows Computer is by installing VLC Media Player. It is free and works great with FLV files. Windows Media Player does not come with the right Codec to play FLV files and finding it can be a hassle.
To Play FLV files download and install VLC Player from the link below
https://www.videolan.org/vlc/index.html VLC Media Player Download
Then go to the FLV file you want to play and click properties and change the default program to VLC Player.
If you do not like VLC player you can also try out Media Player Classic both programs are free to use
https://mpc-hc.org/ Media Classic Player Download