It's not possible. DVD+/-RW discs are made so they can be erased and re-written. DVD+/-R discs are made to be writable only once. There is no way to switch between the two formats beyond simply buying a new DVD.
If the reason you're asking is something along the lines of needing some burned video to work in a DVD player that does not support playback on DVD+/-RW discs, you'll likely need to just get a DVD-R instead. If you're determined to make it work with the RW though, make sure you finalized the disc when you burned it. This isn't a sure-fire fix since some players simply won't read off of an RW but it can't hurt to try (though it may render the RW useless in the sense of it being re-writable).