I've been downloading Dota2 reborn and when it reached nearly 55% I exited Steam as I had to download another program. When I reopened Steam the download starts again from 0% and due to my slow internet connection it would take like 3 hours to reach 55%. So is there any way to resume the download where I left off?
Steam resumes your downloads regardless when and how you interrupt it. Restarting at 0% is an error. Try to cleanout the DOTA2 steam directory and start your download. Stop it at lets say 5% and than resume it. See if you can resume. If this doesn't work I would probably reinstall steam or ask steam support. Resuming downloads is a basic feature of steam.