I have a 2010 Toyota Sienna and the heater is not heating as it should. What may be the problem?

Is the engine up to temperature? If not, it could be a open thermostat. If it is hot, your water pump may not be pushing enough coolant or the heater core is clogged.

or...the valve that allows hot water in to heater core is faulty, not opening, either because of the linkage to it or the valve itself. 2010? Warranty? But is seems you have two problems...cold air and not enough flow..both...makes me wonder if something is obstructing the heater core air flow, like a piece of paper even, big enough to cover all or most of the core. Try the A/C for a minute see what happens

Both of the above answers are good advice. The other area to check is the control box usually under the dash which has flaps which control where the air goes, and some method of controlling those flaps. Even if your heater core is getting warm water from engine, if the air is not directed over the appropriate area, this could also be an issue. Fan is working right?