Update a package that is not in AUR

First uninstall old package.
Put the PKGBUILD and the package.install file in one folder.
Open terminal in that folder and do:
makepkg -sci PKGBUILD
This will build and install the new package.

And here is an example for adeskbar 0.3.8:
the new PKGBUILD
and adeskbar.install for 0.3.8.