The best practice for exchange is to have your log files on a different drive or set of drive spindles (RAID array) than your data bases. You will need to install additional drives for your log files, preferably in a RAID configuration.
Once you have the drives installed, then you can change the log file location at any time. Go to the EMC, expand the server configuration and mailbox role. Right-click the storage group you want to move and choose “move storage group path” then browse to the location of where you want the logs moved to.