If you need to restart your nodes, simply closing your terminal tab where you are nodes are active, won't work.
6.1 In order to "kill" your nodes, execute the following command in a new tab
6.2 If you are encountering a problem, especially when upgrading to a new version of the cardano-node, you may want to delete the database, so your node can fully resynchronize with the blockchain. You can do that by deleting the databases of your nodes
rm -r block-producing/db
rm -r relay1/db
rm -r relay2/db