At the top of the window appear the current filename and the minimum, maximum, and mean values from the image file header. If there are multiple image files loaded, these entries will be updated when the current file is changed.
The Lower limit and Upper limit are used to scale the image pixel values linearly to the 8-bit display; values between the lower and the upper value are scaled to the range 0 to 255, and values outside this range are truncated as black or white. When you first open the window, these values are set based upon the current position of the contrast sliders, so that if the image is reloaded, it will have the same apparent contrast with the sliders set to 32 and 223.
The Apply button will reload the image with the given limits AND set the contrast sliders to 32 and 223.
The Calc button will recalculate suggested lower and upper limits based upon new settings of the contrast sliders. Thus, you can either type in new lower and upper limits, or use the sliders and Calc button to set the limits.
The Just Reload button will reload the image without changing any scaling or slider positions. This is useful if you have replaced the image file and want to reload from the new file but want to keep the scaling the same.
All data for the current file are reloaded with the given scaling, unless data are loaded into the image cache and the cache is not being kept full. In that case, the data in the cache are flushed but only the current image will be reloaded with the new scaling.
Replacing a file and reloading it here should work as long as the image dimensions are the same in X and Y. If the new file is larger in Z, the additional sections will be ignored; if it is smaller, only the sections within that new range will be replaced. If the size differs in X or Y, it will make a mess of the image. In Windows, it may not be possible to delete the existing file or rename it to a backup while it is open, but programs like Newstack should be able to write new data to the file.