You need to be on Amazon Prime trial membership to extend trial time.
At anytime before your trial period ends, go to
Step 1 Your Account > Manage Your Prime Membership
Step 2 Click on Change Payment Option:
Step 3 Change to the other option: (from monthly to annual, or annual to monthly) do this two times as each time this will increase your trial by one additional month.
Step 4 Congratulations, You've unlocked an additional 30 days of these exclusive benefits:
Note: Amazon will not charge your card until after your trial expires.