Can a dental retainer improve an overbite?

No: Orthodontic retainers are use to hold teeth in their position. Dental appliances can move teeth. To improve an overbite orhtodontics were the teeth have braces on them is your best choice to improve an overbite.

Yes, but...: The term "overbite" means different things to different people. If it's a true overbite, where the upper teeth excessively overlap the lower teeth, then you do not want to treat it with just a retainer. You should not cut corners....It may cause problems later on. See a fully trained orthodontist. You probably need full orthodontic treatment with braces on all your teeth.

Yes: In a growing child, intercepting an "overbite" is excellent preventative treatment that often can avoid comprehensive orthodontic treatment involving pulling permanent teeth, headgear treatment and even jaw surgeries. This is call interceptive orthodontics and the proper diagnosis combined with the proper "retainer" or oral appliance called a bionator can produce excellent results.

Retainer: Retainers are usually used to stabilize ones bite after braces. There are special retainers that can improve minor crowding but they are not intended to correct ones overbite.

Not a retainer: Retainers hold teeth where they are. There are removable appliances that can, if used at the right time, reduce the severity of some malocclusions in growing children. See a fully qualified Orthodontic Specialist for an (usually free) Initial Examination and discussion of your treatment problems.