re-orientation to 4.7.2

i have just had an upgrade to 4.7.2, after running a very old version into the ground. it seems most of the content has survived the shift but i'm having trouble re-orienting myself to the new environment. i'm using a default theme, which is ok for now, and have made some rudimentary changes to the template but have got stuck on being able to make changes via editing files. i used to be able to ftp new files but not sure know how to do it. i have been trying to sort things out via the forums but if their is a gentle soul willing to hold my hand and get me started i would be grateful. i have some basic experience with html, css and php but can pick stuff up pretty quickly. my site is merriaction.org cheers, david