Updating the PLC project on the PLC

TwinCAT basically offers two options for transferring a modified PLC project to the controller: download or online change.

A download leads to a new compilation of the PLC program. In addition to a syntax check, program code is generated and loaded onto the controller. This will stop the running program. A download is the recommended type of data transfer, since it always leads to a defined output state as a result of the program stop and the reinitialization.
During an online change, only the modified parts are reloaded into the controller. A running program is not stopped. The online change option should only be used after minor modifications of the PLC project. After more extensive modifications, the program behavior cannot be predicted reliably.

See also:

TC3 User Interface documentation: Command Download
TC3 User Interface documentation: Command Online Change
Additional information
Performing an Online Change
Execution of a Download