Replacing front chainring on my old bike I have an old Giant Peloton Lite. Certainly considering its age, it's a very nice bike. The front chain ring is a bit worn out, however, and I'm looking to replace it.
I have a few questions about that:

It seems like a relatively easy job. Screw the old one loose, put the new one on. I've seen a video about it, which seemed to confirm that. I like doing small maintenance, so I like to do things myself if possible (instead of taking it to the local bike shop). Is this true, or is it really something I should have my LBS do?
As for parts: my current ring is a 53-teeth, I found a 50-teeth in the same make. Can I use that, or will that give me unforeseen issues?
In general, what are things to look out for in chainrings? I'm very new in learning about these things, so I'm curious what are the relevant factors determining my bike's compatability with a given chainring. The chain, for sure. But can one match a chain ring with a very different cassette or freewheel, provided they run on the same type of chain?
It is generally recommended to change the cassette (or freewheel) if  you are changing the front ring. Is that really necessary? The freewheel is still really good.

I know these might sound like naive questions, but, I'm learning and having fun doing that. Thanks for your help.
(To further specify: it's a 2x6 Exage 500 drivetrain)
Chainring replacement is pretty easy. It's easier if you take the crank out of the bike but you may be able to get the rings over the spider and crank arm without removing the crank.
You can find instruction videos for pretty much every bike wrenching task. Here is one for chainrings.Note you will need a special chainring bolt tool to fit in the long thin slot on the back of the chainring bolts.
Just get a replacement chainring from a known 3rd party manufacturer, these are not hard to find in the major online bike component stores. You obviously need a ring that has the correct bolt circle diameter (BCD) and bolt pattern.
If you have 53/39 chainrings and you should replace them with the same size. 50 tooth rings are designed to be used on compact chainrings with a 34 tooth small ring. If you fit a 50 tooth you'll have a large reduction in gear ratios on the big ring and a smaller ratio gap between the big and small rings. Older cranks with 130mm BCD won't take small rings smaller than 39 teeth.
Sprocket and chainring wear is accelerated by worn chain 'stretch' (the bushings wear allowing each link to move slightly further apart from its neighbors). If you have worn you chainring out chances are your chain and sprockets are worn too. It's a good idea to replace the freewheel, both rings and chain all together. A worn chain on new sprockets or rings has a tendency to slip over the teeth.