Hi,
Try the options in this [http://visihow.com/I_do_not_remember_my_Google_account_username_or_password|link] first before doing a hard reset.
If they don't work then here is a link which shows how to [http://www.hardreset.info/devices/lg/lg-f510l-g-flex2/|hard reset] your phone.
Be aware that by doing a ''hard reset'' you ''will erase all your data and downloaded apps''. it will restore the phone to its' factory default condition (or last successfully updated OS condition).