It depends on why the coverage is fluctuating. If you are in a region of bad coverage there's nothing you can do. If there is a network problem you'll just have to wait until it's fixed, though you could report it to your carrier. If the phone is actually defective then a repair is needed. If you have a dying SIM card then replacing that will fix it.
To say more we'd need enough information to guess at what the problem might be. If you want to post again with more information, such as whether the phone has always been like this, whether other phones are OK in the same region, whether your phone has had any updates, whether you've tried the SIM in another handset (if you aren't using a CDMA phone), whether the problem is geographically localised, whether it's just data that drops or the signal drops completely, then we might be able to suggest something more definite.