In recent years, the development of digital twins for robotic and rehabilitation systems has gained increasing attention due to their ability to provide accurate virtual replicas of physical devices for analysis, testing, and control. This thesis presents the design of a digital twin of the FloatEVO upper-limb exoskeleton, developed at the Rehab Technologies Lab of the Italian Institute of Technology (IIT), using the Multi-Joint dynamics with contact (MuJoCo) simulation environment. FloatEVO is a 7-Degrees of freedom (DoF) robotic exoskeleton designed for upper-limb rehabilitation and assistive therapy, targeting both the scapular and glenohumeral complexes as well as the elbow joint. Its structure reproduces the main articulations of the human arm, allowing for natural range of motion and for the possibility of implementing multiple control modes, including assist-as-needed rehabilitation exercises.