An inexpensive replacement tub may be narrower than the old tub. Many people find a narrow tub uncomfortable, so buy one that's comfortable. Most tubs fit into a 60-inch opening, but some older ones may be longer. Measure to make sure your replacement tub will fit.
Some spa or whirlpool tubs fit a standard tub opening. Installing one is not much more work than installing a standard tub. A spa or whirlpool needs to plug into a GFCI (ground fault circuit interrupter) electrical receptacle.
Preparing the floor and the walls
If the bead of caulk at the base of a tub has even a small gap, water that puddles on the bathroom floor will seep underneath the tub, quickly damaging any bare wood. To be safe install protective flooring on the entire floor, including the area the tub will cover.
About a day to install a replacement tub where there is an existing drain
Drill, torpedo level, screwdriver, strainer wrench, putty knife, pry bar, hammer
Making drain connections in a tight spot, basic carpentry skills
Clear the area; cover the floor with plywood and a drop cloth.
Tub, waste-and-overflow unit, plumber's putty, pipe-thread tape, cement backerboard and tiles or other wall-finishing material, caulk
Check the drain and replace any damaged parts. Consult the manufacturer's literature and measure to make sure the drain is in the correct location. Purchase a waste-and-overflow unit and determine how you will connect it to the drain line. Screw ledger boards to the studs at the height recommended by the manufacturer. Ideally the finish flooring material should run under the tub.
Before tilting and moving the tub, plan the move so you avoid damaging the waste-and-overflow unit, which protrudes below the tub. It may work best to rest the tub on 2x4s part of the time. Move the tub into position with a helper. You may have to tilt the tub. Slide it into the opening and gently lower the tub in place. You might want a helper to guide the overflow tube into the drain line while you do this. Slide the drop cloth or any other protective material out from under the tub. Protect the tub interior.