Hi,
When you insert an Opportinity it should default to the account account manager unless changed. However, if you want to update all records to match then you can run the following SQL update:
Update OPPORTUNITY
Set Opportunity.accountmanagerid = Account.accountmanagerid
FROM ACCOUNT LEFT JOIN OPPORTUNITY ON ACCOUNT.ACCOUNTID = OPPORTUNITY.ACCOUNTID
Where Opportunity.accountmanagerid <> Account.accountmanagerid
Backup the database first!
Regards, Adam Travers