Can you edit your website yourself?
Of course.*
The asterisk
We'd strongly recommend hiring a professional unless you're proficient in HTML and CSS. Just like we'd hire a plumber to fix our plumbing or an electrician to fix our electricity.
There are programs out there that help you edit HTML without knowing any code. Dreamweaver is an option, but can write some pretty muddy code, and Dreamweaver has a hard time understanding anything that wasn't written in Dreamweaver. Adobe continues to support a program called "Contribute" that is supposed to make editing websites "fool-proof". 1) nothing is fool-proof; 2) Contribute is less-intuitive than Dreamweaver — which is saying a lot considering Macromedia's legacy user-interface; and 3) to do this, you have to design in Dreamweaver and use their template system.
The other trick is that a website is a lot more than just HTML and CSS. To really edit a website, you're going to need to know Photoshop, Illustrator, and possibly Flash — and you'll need to know all of these programs to a fairly competent degree. And you'll need the fonts used — fonts are like software, everyone needs their own copy, typically a few hundred dollars on the low-end.
Assuming you have the $1500+ software and etc. to edit your site, there is still time. A professional designer writes HTML and CSS at least once a day, usually for 8–14 hours at time, that's what makes us good at it. If you're just jumping into your website once a week, it's going to take you a significant amount of time and there's a good chance something can get messed up. Furthermore, a professional is testing your site on several platforms, with several browsers, and is watching out for hundreds of things that can fall through the cracks.
Other options:
It's actually cheaper to just hire someone who uses HTML and/or CSS for a living. And you'll save yourself a lot of headaches. Barring that, if you have to control the process, we'd suggest a content management system (CMS). There are downsides to this too, it does lock you into templates and therefore hinders your options a bit, but that's a tradeoff. And it costs a bit more to design a website with a good CMS.
We work with several clients on retainer to manage their website as well as to continue to develop it. Unlike print, the beauty of a website is that it's a constantly evolving organism, and it's never really "done".
The real issue
Your website is a marketing tool. It should pay for itself, hopefully exponentially. If it's paying its own way, then it suddenly seems a lot sillier to risk the integrity and professionalism of your website because it's fun to edit.
At the end of the day though, yes you can edit your own site.
