If you are serious about gathering data about your website then it's likely that you will be using Google Analytics to do so. And if you are selling products or services from your site then tracking Goals and Goal conversions will be nothing new to you.
I've recently set up a new goal that I then forgot to activate, and it took me the best part of 10 minutes to figure out how to edit an existing goal in Google Analytics and to switch it from inactive to active.

Since I know that I will be Googling for this exact same issue again in about 2 years time once I forgotten all about it I am now documenting it here (note to future self: see, I knew you'd be coming along again).

1. Log into your Google Analytics account and navigate to the website profile for which you want to edit an existing goal.

2. Click on 'Admin' at the top right

3. Click on your website property label

4. Click on the Goals tab

5. Use the toggle switch to enable the goal

Hope this helps someone (I know it'll help me one day...).