your looking at a full install if you want something at least half decent these Gen 2 Hondas suck for sound quality..look through the forums under sound and audio to give you an idea on what to do. Welcome to the forums btw !
Head unit is near-impossible to replace without very extensive dashboard modding, and factory head does not carry any preamp outputs.
My own audio overhaul of my 2013 EX has been in two steps. One to upgrade the sound quality, another to upgrade the convenience factor.
Tricky part of Pilot audio is that is has built-in sub amp and internal crossover and it forwards the low frequencies to the sub. If you try to amp off the usual 4 channels, a lot of your lows will be missing.
Part 1: Connectors. I think your 2015 should be about same as my 2013.
Your 24 pin (A), leftmost if looking at back of audio unit.
Female on wire side, locking tab on top, top row left to right is pins 1-12, bottom row is 13-24
Rear right 6/7, rear left 10/11, front right 18/19, front left 22/23
Accessory (purple) is 14,
Ground and +12V are 12(black) and 24(white) (larger sized pins)
Connector E, 16 pin, next to antenna connector,
wire side female, lock tab on top, upper row is 1-8 left to right, bottom is 9-16.
pin 1 (white) subwoofer +, pin 9 (red) is sub-.
To make sure, I recommend getting a speaker tester. I got an 'InstallBay IBR68' from Amazon.
Part 1: AudioControl LC2i line converter. I connected HU sub output to left speaker terminals, and split off rear right speaker to right terminals. Power LC2i from HU Ground and +12V pins, it only needs 150mA. Enable GTO, run Remote Out wire to amp, and 'Bass Output' dual RCA to amp.
Part 2: I used Rockford Fosgate R600X5 amp, mounted under passenger seat. 5 channel mode, RCA from LC2i go into sub inputs, and speaker outputs from HU go into line level inputs. Remote out from LC2i goes to the amp remote. LC2i senses incoming signal and powers on, also sending a 12V signal to the amp.
Part 3: Ran new speaker wire into all 4 doors from the amp. You CAN run wires from amp back to dash and hook them into vehicle harness so you can use existing speaker wiring. I wanted beefier wires and direct runs. Speakers: Focal Integration ICS 165, installed using HushMat black foil pads.
Part 4: Ran new wire from amp and replaced factory sub with a Kicker CompRT 42CWRT81 8" sub. Also used HushMats.
While I had everything apart, I ran an additional pair of RCA cables for when I would be installing a headunit with preamp outputs.
Took some time to tune bass compensation in LC2i and setup the amp.
Massive improvement in sound quality.
Second part will involve modding the dashboard to put in an Alpine ILX-207 in place of the display screen. It will require some extensive playing with wires to assure the rear camera and built-in mic work. No idea when I get to that, though...