Mark's suggestion about using squeeze would be a reasonable suggestion on certain platforms (such as the 1700 series of routers). But as you discovered it is not a supported command on the 1800 series of routers.
You could erase the existing image and then load a new image. But before you do that I would suggest that you check the output of show flash on the router and see what is taking up space and see if there are things that you might delete without having to remove the active image.
The good news is that once the image is loaded in the 1841 the router will run it without needing it is flash any more. So if you delete the image in flash the router will continue to run just fine while you load a new image. The risk is that if something causes the router to reboot before you complete getting the new image into flash (or if there is some problem with the new image) then you have no fallback.
use "sh flash" to find out the existing image name. Then use "delete flash"filename" to delete the image. After doing this do not reboot the router until you load the new image. If you do it will boot into rommon. If that happens you can still load the image via TFTP in rommon.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...