Welcome to LQ....!!!!
Did you Google this, as I can see many blogs/sites suggesting couple od ideas. Moreover, provide info like :
a) what wifi USB adapter you have (like TL-WN8200ND...etc)
b) what is the purpose of this adapter (like improving signal strength,...etc)
c) how does your router connecting to this adapter
d) any other component involved (like High Gain ANtenna,....etc)
Anyways, pls refer this youtube vedio and a reference blog link to configuring wifi USB adapter, as these seems helpful :