Wardrobe update ideas from top American designers

How would you like to give last year's dress or suit a new look for $1.29? It can be done. All it takes, with many outfits, is a pair of shoulder pads.

For another $2 you can update everything in your closet with a pair of colored stockings. Or, as several designers suggest, a long, slim skirt that will team with your blouses or jackets will also give your separates a fall '82 look. And with a bit of shopping around, you may be able to find the skirt in this season's silhouette for as little as $20.

These are just a few of the fall fashion ideas offered by top American designers when asked: What single item would you suggest for a woman on a clothes budget, and how can she update what's in her wardrobe?

Alfred Fiandaca: ''A pair of colored stockings is one way of doing it. Or even a pair of sheer stockings over opaque ones is interesting. This creates a pattern and looks wonderful with a tweed skirt and cardigan, for example. Color is very important, too. I just got back from Europe and found that countries such as Italy are bombarded with color. So a woman could buy two or three inexpensive bracelets in bright colors or a pair of shoes in a bright color. If she's going to buy a dress, I think a T-shirt-shaped dress is an idea; and she can belt it if she wants to.

''These dresses are available in a full range of prices - right down to about lots of things in the wardrobe.''

Betty Hanson: ''I recommend a slim, long skirt. Women haven't had a new skirt for a long time. Last year they were buying pants, which were the key fashion then. But now pants - basic, classic pants - are taking their place in the wardrobe as a staple and skirts are zooming. They're the newest thing. They shouldn't be worn too long, either. Remember that you have to walk in them. I like boots with skirts, too, with both straight and fuller skirts. The boots should be high enough so that the hem of your skirt goes over the boot; your leg shouldn't show.''

Halston: ''Perhaps one could buy a few separates that could be changed around to give more options - pieces such as a blouse, skirt, and a jacket. They should be the type of separates that can be dressed up or down. Actually, there isn't one mood in fashion - fashions don't change that much. If you have full skirts in your wardrobe and they look right, wear them. If you like to wear your things short and they look good, wear them short.''

Alexis Kirk (accessory designer): ''A special quality belt is probably the most important single thing a woman can buy. That's because it can pare down a full dress and define the waist, which is a woman's best asset. Belts are collectible pieces as well as conversation pieces. They're an accessory that a man will notice.

''Belts are more than just belts. They're jewelry for the waist. It's like wearing sculpture. If a woman gets a belt with a handsome pewter buckle, let's say, she can then get interchangeable straps in different colors and textures. This type of belt can be worn day or night.''

Bill Blass: ''A straight, lean skirt is the first thing that comes to mind. What length? It depends on the woman, her figure. This leaner look is in the air. It looks right with blouson tops or overscaled jackets. As far as the existing things in her wardrobe, accessories are vitally important. Hats are back - simple, mannish ones, not anything fussy. I'd say a man-tailored hat would update her clothes. Jewelry is another thought. We showed large, modernistic pieces on the runway. Mrs. Reagan was photographed recently wearing a black, plastic, modernistic necklace. It doesn't have to be precious jewelry. But this type of jewelry must be chosen carefully. It takes careful calculation. With some of the futuristic-looking pieces, a woman must be certain that they don't overpower her or overpower her dress. The necklace must work with the outfit. It's a great look for the sophisticated woman.''

Caroline Simonelli (for Mollie Parnis Studio): ''Buy a dress or an outfit that is slimmed down - lean and long . Be sure that it's simple, which can make it timeless. Then you'll be able to do more things with it. You can take the dress and wear it with one of your jackets. Last year's fashions were fuller. This fall, fashions are slimmer.''