As suggested by Horace1, that is the classic symptom of an over-compensated x10 probe and probably nothing at all to do with the oscilloscope itself.
Do this:
1. Make sure the probe is set to the x10 position if it has a switch on it.
2. in the side of the probe you should find a small screw slot. With the display showing as it is in the photographs, turn the screw slowly and you should find it levels the top of the waveform then goes on to make the top slope the other way. The correct setting is when the top and bottom lines are horizontal.
Use a non-metalic screwdiver if possible and be very gentle, if you break it you will have to buy a new probe! They are expensive. See if there is a screwdriver packed with the probe, they are usually supplid with one.
Note that you are matching the probe to the oscilloscope, if you move it to another oscilloscope you should repeat the operation on that one too. With the compensation set incorrectly, any AC amplitude measurements you take are potentially wrong.
Brian.