Ower image file size

Hi everyone, apologies as i know this is a very newb question. Ive bought Gigapixel ad love it. Ihave a few images ive upscaled to put on redbubble and inorder to do so i need to remove the background via Canva. Unfortuantely the maximum file size is 25mb and my upscaled and resized image is now 85mb. How can i change the file size while maintaining the quality? I know it will reduce with the lower size but could somebody please explain the best way to do so. For reference ive upscaled x6 and the dimmensions are 9852x12948pix. Thank you kindly for any advise you can give me.