Cannot change the theme

I know how to change the theme but cannot. I set up my blog a few months ago but did not use it once I had learnt the basics. I have spent several hours today trying to change the theme, watched ‘how to’ videos, visited helpful youtube tutorials, etc., etc. Doing everything right but the original theme I set up months ago, stubbornly refuses to disappear. Anyone got any suggestions as to how I resolve this issue?