Learn Russian: Утром воздух холодный.

To say when a part of the day happens (like "in the morning" or "in the evening"), Russian uses the Instrumental case. The noun утро (morning) gets the ending -ом to become утром. You might remember this same time-of-day pattern from words like вечером (in the evening) or днём (in the afternoon).

Where is the word for "is" in this sentence?

In Russian, the present tense of the verb "to be" (am/is/are) is usually left out completely. You simply place the subject (воздух) right next to the adjective describing it (холодный) and the "is" is understood from the context.

Why does холодный end in -ый?

It is an adjective matching the noun it describes, воздух (air). Because воздух is a masculine noun and the main subject of the sentence (Nominative case), the adjective takes the standard masculine Nominative ending -ый.

Could the word order be changed to Воздух холодный утром?

Yes, Russian word order is very flexible, so that would also be grammatically correct! However, putting the time word (утром) at the very beginning of the sentence is the most natural and common way to "set the scene" before describing what things are like at that specific time.