Congrats on your new toy. They're a lot of fun to drive...but they do require some TLC, of course what car wouldn't that's 30 + years old. I have a 74 914 1.8 as well that I recently picked-up and am working on. It had been 25 years since my last 914.
The dust cap concealing the wheel bearings is just pressed into its cavity. Behind the cap is a clamping nut fastened by an allen bolt. After removing the nut and thrust washer, remove wheel hub/brake disc assembly. "To do this, lay the wheel hub assembly over a clean cloth. Tap the lower bearing out from the inside with a hard-wood stick and a hammer. Turn the wheel hub over. Tap the other bearing out in the same manner. Do not mix up bearings. Note: In some cases it may be necessary to separate the hub from the disc to remove the bearings. Mark the relationship between the two, then remove bolts joining them."
Manual:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=6968907147&category=268