It is not possible to reverse a Gift Reversal. Your only option would be to re-enter the gift. You can, however, change the Reversal Date in the Gift History data grid.
To change the date on a gift reversal find the reversal in the gift history data grid, highlight the line, right click on the line, select 'Change reversal date'. Enter the new date and click OK. You might need to process journal entries to move the revenue and assets to a different time period. They do not automatically create for you. This will only change the date in the gifts module, in order to affect the GL, reversing journal entries must be done.