PHP files are just text files, so you can edit them with any text editor.
However, PHP is a programming language that is used to create dynamic (i.e. changing) web pages. If you don't know how to program in PHP (and given your question, I'm guessing you don't), you may find it difficult to know what to edit to get the changes you want.
It also depends on what changes you want to make. Depending on how the site has been laid out, visual changes (text colour and size, images, text placement) might be changed by editing CSS (Cascading Style Sheet) files (also text files).
Your previous web designer may also have given you the means to add content etc., but you'd have to check with that person.
If you're not proficient in Web design, you're best off getting a professional in - either the person who designed it in the first place or someone else. You can learn PHP, CSS, and HTML (the bare minimum you'll need) from the Web, but that may take longer than you need right now. (But do search for "HTML tutorial", "PHP tutoral" and "CSS tutorial").
