Move the mouse pointer to the data plot to highlight (yellow). When right clicking and holding, a pop-up (context) menu will appear. Select from the context menu Select Data for fitting. A new tab with the name of the data selected will appears. Select a model used for the fit from the combo box. Model's parameters will be display.Set initial parameters if need. Check at least one parameter to fit.Click on Fit button. New parameters values will be displayed as well as error and new chi square associated with the fit operation. The model will be replotted with its new values. Errors will be display on the statusbar. Single fit can also be perform on the "simultaneous fit " tab is parameters are previously selected on the page corresponding to a model name check. if one model is check, click on constraint fit button to perform a single fit. results will be displayed on the appropriate tab.