In Edit mode, open the Individual Datasheet by clicking on the tab if necessary, and select the name of the individual to be edited in the listbox on the right-hand side, if necessary typing a search string in the edit field above to find the name.
When you have finished editing details of the individual, click on the Save button towards the bottom of the window, to save the information within the program.
Before leaving View/Edit modes, you should also save the gedcom file to disk, by choosing Save from the File menu, or clicking Save on the toolbar. You will be prompted to do so.