I changed my headset at an lbs and they over looked putting on the top cap the new headset came with[used the old one different brand],so can I just unscrew the the top cap and replace it or will this cause the headset to lossen and need reajustment.It's a threadless.Thanks

yes you can replace. there should be no need for any readjusting to your headset.

If the stem clamp bolts are properly tightened, removing the top cap will have no effect on the headset adjustment.

Great thanks thats what I was worried about,was not sure if it would lossen anything,so that only happens if the stem bolts are lossened first then the top bolt. I guess then if I wanted to aline the stem and not effect the headset you would just lossen the stem but not the top cap.How tight should I rescrew in the top cap bolt?

Once you loosen the stem's clamp bolts, the top cap tightness will vary depending on the headset manufacturer's recommendation.

At the very least you want it tight enough that there is no play in the headset when you clamp on the front brake and try to rock the bike fore-and-aft. Manufacturer's recommended torque for headset's I'm familiar with varies from 8 to 20 inch-pounds.