Open Electrum (Install it using the installation .exe that you have, if it isn't already installed)... then use the "File -> Open" menu option (or click the "Choose..." button if prompted to create a wallet)
This will show you where all the Electrum wallets are located. Open the wallet you want, then use "Wallet -> Seed", this will show you the seed mnemonic for the wallet... IF it has one (imported wallets including imported BIP39 wallets DO NOT store seeds in the wallet file).
would you (or anyone) know what the default wallet files get named?
default wallet name is: "default_wallet"
NOTE: Electrum does not use file extensions for wallets