If you have connected your Lightyear account to the wrong MYOB account, you need to do the following.
Go to Left Hand Nav Bar > Accounting Softwares > MYOB, click disconnect.
Open a new browser tab or windows, and paste this link into the URL https://secure.myob.com/oauth2/Account/Login
Hit ENTER > Sign out
Go back to Lightyear > Admin, and under MYOB, click connect.
Then follow the instructions here on connecting to MYOB Account Right / MYOB Essentials.