There is a manual release screw on the side of the motor bracket that you turn to release the pressure and lower the unit. Once you get it where you want it, tighten the screw back. You may have a bad relay or switch, but the manual release will let you put the motor down. Be careful though, as soon as you release the pressure the motor will drop so stand clear of it.
On my Yamaha it is one the left side, but you need to check the manual to see where it is on the Mercury.
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