remove your backseat. under the backseat you'll find an aluminum pry it up & under it is the fuel pump access. remove the bolts and lift your pump out the tank. follow your new pump installation instructions and reinstall everything.
Not a simple answer as you would think. Depends on the year model. Although standard is 14.8 gal they also had a large size of 17.2 gal. Now if your model is an early rear , say, 1981 then the size was 13.7gal. The 2001 Toyota 4Runner has a fuel tank size of 18.5 gallons.