Firstly, I would recommend that you upgrade Electrum to at LEAST v2.9.3 (for Windows 7/8 or earlier)... and v3.0.3 (Windows 10, Linux or OSX)... (do NOT try and use v3 with Windows 7/8 or earlier, it probably won't work!)
Then you run Electrum and if prompted to create a wallet file, select "Standard -> I already have a seed".
If you're not prompted, and it opens up into the application, you should use: "File -> New\Restore -> Standard -> I already have a seed."
Once you are at the seed entry screen, input your 12 word seed, select a new password, and your wallet and all your transaction history and addresses etc will be regenerated!