Go to Settings > Hosting and choose either a monthly option or a 12-month option (the 12-month option is the better value). It's a one-time payment with no subscription or auto-renewal. See current hosting rates on the checkout screen.
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Extensions add time onto your current hosting period. If your hosting has already expired, the new months start from today and your site comes back online immediately.
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You can extend as many times as you like, and extensions stack.
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