In order to rename a node for your graph, you can click a node, then go to the top right menu for "merge" function, click the chain button, rename your node, and then click the plus button. The new name will be saved into the graph:
You might have this problem when a certain word was lemmatized in a way you don't like. In this case, you can either rename the node as above or also switch off lemmatization completely.
In order to switch off lemmatization, set "Do not lemmatize" in the language processing settings of the graph by following the How to Change Your Language Setting procedure.
Another way to avoid lemmatization is to mark the word that you don't want to be lemmatized (or that you want to have a certain spelling unchanged) with a hashtag by using the search and replace function in your text editor. Then importing this text to the graph and setting to process both hashtags and words (by default it's either hashtags or words).