Have you considered whether the woman who sold you this antique code knows how to write a modern site? If not, you could waste a lot of time (and/or money) and get no further down the line.
I suggest that the solution really is to learn how to do it yourself. There are HTML, CSS, PHP (and other) reference works and tutorials on the web. Browse a few.
If you do want to try altering the code yourself: MAKE AN OFF-LINE COPY and work on that.
Right from the start, archive everything. I.e. Before you make a change, however trivial, take a back-up of the code AND KEEP IT. You never know when you may want to look up some old code to figure out how it worked. Also I frequently have to restore from the archive, when a change does not work as expected! It's far easier to copy back than undo all the changes!!!
E.g. I have an archive folder for each site, and I copy the whole site into a sub-folder which I name with a name, a sequence number and a one word comment. E.g. If the off-line working copy is in folder .../www/simtime/... it would be archived to, say, .../archive/simtime/sim001_valid/...
a) 001 is the sequence number.
b) "Valid" means I have validated the HTML and CSS with the W3C or WDG validators.
Other comments might be "ok" for a version that appears to work fine, but has not been validated, or "duff" (yes I archive the failed changes before I step-back to a previous good version)!
- I use Crimson Editor v3.60, and am very happy with that. Note: I've had problems with later versions of CEDT on some versions of Windows. V3.60 works fine for me on XP, Vista and 7.