[yocto] Update a recipe to use newer sources

[Arno Steffens]

Can you please help me? 
For one or 2 packages I need newer sources than available via yocto. Is there a guide what and how I have to modify (or better copy, modify and add???) a recipe where the only change is a newer source code release? I am aware that this can create trouble, but sometimes I need a bugfix that it is only in recent versions available.
Thank you
Arno