-Open your SharePoint Central Administration site.
-Select the Monitoring section.
-Under Reporting, select Configure diagnostic logging.
-Configure Logging and specify a local path to use for the trace log on all servers in the farm. The default location is C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\LOGS.
-To change the location, type a new one in the Path field. The location must exist on all servers in the farm.
-Next specify the number of days to store log files.
You also have the option to restrict the maximum amount of storage to use for logging.