You can use the Insert Function button in Excel 2016 to edit formulas that contain functions right from the Formula bar. Select the cell with the formula and function to edit before you select the Insert Function button (the one sporting the fx that appears immediately in front of the current cell entry on the Formula bar).
As soon as you select the Insert Function button, Excel opens the Function Arguments dialog box where you can edit its arguments. To edit just the arguments of a function, select the cell references in the appropriate argument's text box (marked Number1, Number2, Number3, and so on) and then make whatever changes are required to the cell addresses or select a new range of cells.
Excel automatically adds any cell or cell range that you highlight in the worksheet to the current argument. If you want to replace the current argument, you need to highlight it and remove its cell addresses by pressing the Delete key before you highlight the new cell or cell range to use as the argument. (Remember that you can always minimize this dialog box or move it to a new location if it obscures the cells you need to select.)
When you finish editing the function, press Enter or click the OK button in the Function Arguments dialog box to put it away and update the formula in the worksheet.