UNO Supply volts change

G'day, My new UNO's on board power supply creeps up from 5V to 5.6 after a few minutes. Evering thing works OK. Is this is a common fault, only unpacked it a few days ago. What is max volts the UNO electronics can handle. Tks

Is that being powered via USB or an external power supply attached to the arduino power connector?

AVR chip max recommended voltage is 5.5vdc, absolute voltage limit is 6.0vdc.