Synth bodyparts that are exchangable are called modules. Synth modules can be anything from exchangable skin, limbs, parts of limbs or fashion items to internal parts, like organs or sensors. Since Synths are highly modular and the exchanging of modules is seen as a daily routine, modules are often handled similarly to how one would handle clothes. A Synth typically owns a multitude of modules and keeps them at their home, safely stored away in a closet and always ready to be attached or exchanged.
External modules are typically easy to install or uninstall, which means that Synths don't need any help with attaching them to themselves in most cases. Some modules, especially internal ones, are much more difficult to exchange. This is typically done by professionals at a clinic.
While many modules serve fashion purposes, the majority of modules serve professional purposes, in that they will enable Synths to fulfill tasks they otherwise would not be able to fulfill. For example construction Synths are typically required to make use of construction tools and powerlifting modules, which they equip at the beginning of their work shift. Surgeons benefit from finely tuned surgery tools integrated into their arm modules and pilots typically have improved sensors and interfacing methods for improved control over the air or spacecraft.
Modules are produced through private companies and have to follow rigid production standards. Interfacing methods between modules are standardized for maximized interchangability.
Modules sold through unlicensed vendors are decriminalized throughout the Outer Rim and the New Colonies, but pose a significant health risk due to a possible lack of production standards. Purchases of unlicensed modules therefore happen at one's own risk.