I just replaced the 11-year old Garmin on my 2007 2575RL with a Humminbird Helix 5 (what kind of boat do you have?). Everything listed below you could do alone, but its a LOT easier with someone to help - eliminating the back-n-forth. Also, my boat was on a trailer. Not sure this can be done on the water.
The biggest challenge was feeding the wire from the transponder on the stern of the port toon up to the console. My boat is fully skinned, so although I could see from the stern where the wire turned to go to the console, I couldn't get to it.
I had hoped that I could attach the new wire to the old one and pull it through, but the old one wouldn't budge - probably zip-tied somewhere I couldn't see it.
However, knowing that the structure under the deck and above the skin consisted of transverse aluminum beams, I figure that the wire ran thwart-ships between the beams. Also, at the very outer edge on each side, there was a 1.5 inch gap where I could see those beams. On the starboard side, looking through that gap, I could see where the cable bundle came up through the plywood deck into the console
So, I used a cable snake fish tape ($12 at Home Depot) (see below)
- From the port side, opposite the console, I pushed the tape across to the starboard side, where my wife grabbed it with a pair of needle nose plyers. I attached a strong string to it and pulled it back through from the starboard side (under the console) to the port side, and left it hanging out of both sides under the bumper.
- At the stern of the port toon, I fed the tape through the hole in the bracket at the top , and pushed it all the way forward to where the string was (it helps to measure out how much tape you'll need to get to the string). Using a bent wire hanger, I hooked the tape and pulled it out the 1.5" gap to where I could tie the string to it.
- Then I pulled the string back to the stern and out the hole. So now, I have a string leading from the stern of the port tube, up to opposite the console, across the boat inside the skin, and hanging out the starboard side at the console under the bumper.
- I squeezed into the console opening (I'm 6'2" and 225 lbs), pushed the fish tape past the cable bundle, and again my wife grabbed it with needle nose plyers. She tied the string to it, and I pulled it up through the opening.
- Back to the stern: I tied the string to the end of the transducer cable, plus I taped it up tight with duct tape so there was nothing to catch on anything.
- Measuring from the transducer to the 90-degree turn, I wrapped that stretch of the transducer cable with electrical tape for about 3-feet to make sure I didn't cut it on any sharp aluminum at the bend.
- Pulled the transducer wire all the way through, up through the opening to inside the console. This was a bit harder than it sounds. Lots of jiggling and pulling back and forth with the help of my wife (who is now on the org-chart as 3rd Assistant Engineer, in addition to being Chief Mate).
At this point, installing the unit on the dashboard was relatively easy. Just used the old power wires, which already had the recommended in-line fuse. Had to cut the fiberglass and the leatherette cover, which is nerve-wracking, but it went well.
Currently trying to tweak it into shape to get full readouts at speed etc (cuts out over 10 mph).
Good luck. Be patient. You'll learn a lot about your boat!
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I found a few dropped pieces when I took mine off. Removing them should give you all the access you need as you’ll be able see right across the boat and then work everything back from there. 
