In device portal I changed resolution my raspberry pi 3 on 800x600 (60 Hz). After turn-off and turn-on my raspberry pi 3 wont start. Green diode light all the time and on monitor I see Gray stripes for few sec than monitor lose signal. I waited few minutes but my PC also does not see raspberry. Can I revert changes/change setting in same config file or I have to format and install OS again?
Yes, you can change HDMI settings by edit config file.
Revert resolution setting to default you need make sure hdmi_mode and hdmi_cvt with same value with the following ones:
hdmi_mode=87
hdmi_cvt=800 480 60 6 0 0 0
Note: If you use os version 15063 or later, config file content is different from earlier version(14393 or earlier). If there is no hdmi_cvt setting you can add it.