This is what to do:
Name your wallet or go by the default name -> wallet with teo-factor authentication -> I already have a seed -> input the seed phrase -> disable 2FA
Can I do that using the existing Electrum Android wallet app?
I tried earlier, but after entering the 12-seed phrase, the finish button is not clickable.
Is there a step-by-step guide to follow?
Yes. You can use the existing wallet app. Click on your wallet name at the upper right corner -> other wallets -> create wallet.