If you upgrade your membership tier (e.g. from a "Starter" to an "Insider"), your new membership will start immediately and any remaining credit you have on your previous membership will be credited towards the cost of your new membership.
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For example:
If you decide to upgrade from Starter Monthly ($49 per month) to Insider Monthly ($99 per month) halfway through the month, the unused value of your Starter Monthly membership ($24.50) will be credited towards the balance owing on your new Insider Monthly membership ($44.50) and you will be directed to pay the difference ($20.00). When your membership is set to renew you will be charged the full amount of your new membership price ($99.00 per month).

Head to My Account to upgrade your membership.