Unfortunately, you can't but what you can do is ask your supporter to extend the end date so it finishes at a later date.
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If you are using our Custom 1-Click Fundraiser then this can be set within the campaign setup itself. However, if updated AFTER supporters have clicked the link to create a fundraiser this will not update those fundraisers already created retrospectively. If in this instance you needed the end date extended you would need to ask your supporters to do so.