Rename component changes the name of the component/reaction constituent everywhere it appears in a model. For example, if you want to rename substrate to Nitrobenzene, select Rename Component from the Dynochem ribbon.

Selecting this, will open the Rename Component window that contains a list of all the components in your model. There is a box below this, called New Name which is for typing in the new component's name. Click Rename to rename it, or cancel to keep the original name.

Shown below is how to change the name of a component throughout your Dynochem model.