If you're using the Upwork for Clients Mobile App, you can click on the round image in the upper left corner, go to Company Profile and then tap the pencil icon in the account section. On the next screen, you should be able to click and upload the image.
If you're using browser version of the website, it would be on this page. Similar to above, you'll first need to tap the pencil icon and then click on the image to update it.
In both cases for security reasons you may be asked to re-enter your password to access your Settings as that area of your account contains sensitive information.
Let us know if that still doesn't work.
Could you please try to upload your image via computer and also make sure that the size of the image 250x250? If you`re still experiencing the same problem after this, let us know here. Thank you.