Hi Vic is this the 20/25hp mercury?The gearbox needs to be removed regardless, if you can let me know hp and model I will try to assist
Many Thanks Orca

Hi Vic, the manufacturer would be a great help but I will take a guess and talk you through a generic proceedure, you need to remove the 4 or 6 visible bolts around the gear housing where the gearbox attaches to the mid section remove the trim tab above the prop usually accessed via a rubber plug at the top or a bolt through the bottom of it,under the anode you may find a bolt/nut undo this and this should allow the gearbox to come away from the engine (please note depending on the make of the engine there could be something holding the gear shift rod and you will need to come back to me for further instructions) once the gearbox is lowered you will find a housing usually secured by 4 bolts remove these and slide housing over the driveshaft which will give you access to the impeller, slide the impeller off the driveshaft and replace it, reinstallation is opposite of removal, pleas find attached pic of an exploded view of a gearbox to give you an idea of what is involved.
Many Thanks Orca

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