Do you mean the beater bar belt or the motor shaft belt? If so, you need to set the vacuum in a horizontal configuration and turn the vacuum upside down. Next remove the five screws from the base plate and lift the base plate off.
To replace the motor shaft belt, remove it from the outer pulley and the motor shaft. Put the new belt on the motor shaft and the outer pulley. To replace the beater bar belt, release the motor shaft belt to allow access the beater bar. Lift up the beater bar assembly to remove the beater bar belt. Replace the beater bar belt. Replace the beater bar assembly. Replace the motor shaft belt. Turn the beater bar assembly by hand to check that the unit is seated correctly. Put the base plate back on the vacuum. Align the tabs on the base plate with the slots on the vacuum and replace the five screws.
The manual for the Panasonic MC-UG304 is available here: https://www.manualslib.com/manual/940210/Panasonic-Mc-Ug302.html
. See page 5 for images of the steps.
I hope this helps.