When you receive money for a service to be performed later the journal entry will be:
Debit Cash for the amount received
Credit Unearned Service Revenue

After you provide the service the journal entry will be:
Debit Unearned service Revenue for the amount earned
Credit Service Revenue for the amount earned.

A prime example is for a magazine subscription because you pay for your magazine in advance for a certain time frame.

From your question I take it that you are given an amount of Unearned Revenue at the beginning of an accounting period and another amount at the end of an accounting period. The difference between the two amounts is the amount of Unearned Revenue that has been Earned, so you need to do an adjusting entry to bring your Unearned balance to the actual Unearned balance.

An example you have unearned advertising revenue of $1,000 and at the end of the period you still have $200 of unearned revenue. First you need to calculate how much unearned revenue is earned during the period. So $1,000 - 200 = $800 of unearned revenue that has been earned. Now that you know how much unearned revenue was earned during the period you can now do the adjusting entry to bring your unearned account balance from the $1,000 balance to the actual balance of $200. Your adjusting entry will be:
Debit Unearned Advertising Revenue for $800
Credit Advertising Revenue for $800

How do I post this in an adjusted trial balance?

Your adjusting entry will be:
Debit Unearned Revenue
Credit Earned Revenue

This is to get your unearned revenue that is earned during the period that has not been previously recorded as earned revenue.

The adjusted trial balance is done after your adusting entries have been made and is done to make sure your credit account balances are in balance with your debit account balances.