The problem with these heaters is the ambient inlet water temperature. In winter, cold water, you need a lot of power to get it up to a comfortable temperature. Mid-seasons, warmer inlet water, output too hot so you mix cold at the shower. Hot season, like now, inlet water warm, output far too hot, so you mix cold at fhe shower, reducing the flow through the heater ("the slower the flow the hotter the water") and it overheats and trips.....for your protection!
As said elsewhere, yes, open it up....but not until you have isolated the power! The reset will be a tiny grey button on a 4-pronged thingy on the heater element. DO NOT TOUCH ANYTHING WITH YOUR FINGERS,use an insulated screwdriver to click the reset, replace the cover, then switch the power back on. Reduce the power setting until the hot season is over.