Convert pixels to micron

I started using imageJ a few months ago, so I am not an expert on it yet. My doubt is: I have a quantity of images which express cells. I used the default settings on imageJ to calculate the surface area of these cells and then I figured out that I should have measured these in squared microns instead squared pixels.
I must transform the surfaces, perimeter and mean diameter of Feret of these cells of pixels into micrometers in the table excel.
Attached is the image processed by imageJ (1024x768 pixels) ready with magnification 10 and an image for the micrometer slide ready at the same magnification.
if at all possible, can you help me convert these parameters in squared micron instead squared pixels?
if no, how can I calibrate imageJ before processing this image?
Thank you

Please study the ImageJ User Guide and especially section 30.8 dealing with “Set Scale”:

If you want to do the conversion post hoc in Excel you simply need to know the µm/pixel relation that you can get from the imaged scale you’ve shown before.