Example: If you have a channel with 5,000,000 sats on the local side, and a separate channel with a different peer with 5,000,000 on the remote side, you can "pay yourself" 2,500,000 sats, from the first channel to the second channel and will now have two channels both evenly balanced. Sometimes these channel balancing transactions fail because they payments have to go out into the network and sometimes there is not a path. Trial and error is required when rebalancing. To do this in Thunderhub, click the rebalance button in the primary menu on the left side: