customizing header image

I’m brand new to this. Right now I’ve changed my theme to “an ordinary theme” from the default and I want to upload my own photos to replace the default ones. Now I know this there are a million topics on this but not one is there any instructions I can follow in my situation. I’ve tried everything I can, but what I really need is for someone to just walk me through step by step of what I’m supposed to do.
any help is appreciated