In Days Gone your bike is your lifeline. It is your friend, your lover, and the only thing capable of getting you out of some of the horrendously stick situations you are sure to find yourself in. It turns out that simply running away isn't always fast enough, so you really do want to make sure your bike is as good as it can be.
In order to upgrade your bike, you're going to need to put some work in. Each camp has its own level of Trust. You need to spend some time doing missions, handing in bounties, and clearing out Infestation nests in order to build up your Trust. Once you have built it up enough you will be able to upgrade your bike.
Every level you gain in Trust allows for more customisation options, so it is well worth putting in the work to make sure the people of Days Gone like you. Even if you don't like yourself.
Once you have earned your place you can go and see the camp mechanic and have a chat with them. This will allow them to refuel, repair, or upgrade your bike. While the first two are essential if you want to survive your adventure, we are only interested in upgrades for the moment.
You can upgrade the superficial things in the Visual or Paint areas, but for real changes, you want to look at the Performance upgrades. Painting the bike is nice, but flames on the side don't actually make you go any faster.
It is here you can upgrade your vehicle. You can upgrade the engine, exhaust, storage, fuel tank, frame, nitrous, and tyres. Each of these has options within them with more being available the more you bump up that Trust level. It pays to be nice even after the world has fallen apart. Always good to know.