Which parts of my bike should I modify/upgrade?
Should you be looking for comfort, the first part that you should upgrade are the parts that you physically come into with, i.e handlebars, grips, saddle, pedals, shifter.
Should you be looking for convenience, you are advised to upgrade easy wheels, rear rack and more.
Should you be looking to enhance your riding performance and speed, you are advised to upgrade wheelset, groupset, crankset and more.