How to manually rebalance Lightning Network channels?
Lnd itself does as far as I remember not provide an api for rebalancing channels.

However there is the lndmanage repository that uses the lnd api to provide a pretty strong rebalancing tool

https://github.com/bitromortac/lndmanage

Such rebalancing will cost you routing fees as they are regular (circular) payments. 

That being said it is not clear why you would want to rebalance your channels before an onion on them actually fails. There is currently the discussion about JIT routing (which rebalance just in time if a channel such should be used can't be used) and fee free rebalancing taking place on the lightning-dev Mailinglist.