If you start to smell burning rubber from a vacuum cleaner, you will need to replace the vacuum belt. The belt will generate friction against the roller in the vacuum. This means heat will increase, and if the belt burns for too long, it will eventually snap.
When you smell burning, the first thing to do is turn off the power and remove the bottom of the vacuum cleaner. Remove the current vacuum belt and clean all surrounding parts. Throw away the old vacuum belt. It will be most likely be thinned by wear and could be scorched in some places.
Replace the old belt with a new one. It should be reasonably easy to install around the roller. Spin the roller lightly to make sure it turns easily. If the vacuum belt still struggles to move freely, the new one you have put on may burn as well. You may eventually need to replace the roller as well as the belt.
Dispose and Check
Check with your local recycling center to find out how best to dispose of your old belt. Remember that the smell of burning may not always be associated with the belt. Check all parts of the vacuum. A buildup of hair or dirt will increase strain on the motor. This will increase heat to these buildups and also produce a burning smell.