Toyota Shop manuals are not available online AND they are copyrighted anyway which means that they cannot be quoted here.
This process is MUCH too involved and is not a job for an amateur 'backyard' mechanic. However, if you wish to tackle it, get yourself a copy of the official TOyota repair manula or a Haynes or Chilton manual or contact a reputable shop or dealer. HINT: Do not neglect to replace the water pump while you are at it. The engine tear-down for the two components is the same and it will just save you from having to to do it before the new belt wears out anyway.
Belt
Go to 'you tube'. There are several videos showing step by step instructions on there.
The manufacturer recommends that the timing belt be changed at 90,000 miles. == ==
90,000 miles or 150,000 kilometers.
Factory recommended mileage is at 95,000 miles
The 2003 Toyota Highlander has an inline 4 engine.
The 2003 Toyota Highlander is a gas-powered vehicle.
The 2003 Toyota Highlander has a 4-speed automatic.
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The 2003 Toyota Highlander is 15 ft. 4.4 in. (184.4 in.) long.