There is no reason why you cannot replace your own windows if you have the right tools and skills, and most importantly, enough motivation. But it's a big enough project that you should not plunge into it without serious consideration. You may find yourself in over your head, with little recourse other than to hit pause on the project and bring in a contractor to finish the job.
Replacement Window Types
There are two basic categories of windows: new-construction windows and replacement windows.
New-construction windows are complete window units that have nailing flanges or fins on the front that allow the window to be nailed flat to the exterior sheathing of the house. The window unit slips into its opening from outside until the nailing fins are flush against the exterior wall sheathing (not the siding).
Use new-construction windows when you are undertaking major work on your house, such as rebuilding the wall assembly. Outside of major rebuilding, you can use new-construction windows as replacement windows only if the manufacturer allows the nailing flange to be detached.
Replacement windows are window units that contain window sashes inside a small frame. They are designed to fit into your existing window frame. You install them from inside the house, though some exterior work may be required.
Replacement windows do not have nailing flanges or fins. The absence of the flange allows the window to fit straight into the window opening from the house interior.
Use replacement windows whenever the wall system remains intact. If rebuilding the wall, do not use replacement windows.
Installing Replacement Windows
With replacement windows, you don't have to remove or replace the existing interior or exterior window trim. Trim may incidentally become damaged during the project or you may decide that you want new trim. But trim replacement is not required.
With new-construction windows, you have to remove the trim on both sides of the window, and you usually have to customize it or replace it with new trim to fit around the new window. In addition, you have to deal with all of the exterior waterproofing, including installing new flashing above the window, and caulking all of the window and trim joints.
So replacement windows are clearly easier to install, but that still doesn't translate to easy. Removing the existing window while preserving peripherals such as the trim, sill, and drywall can be a delicate process.
Plus, new-construction windows are not off the table. If the wall is in bad enough shape to warrant rebuilding and if you have basic carpentry skills, new-construction windows are worth considering.
Estimate between four and six hours to install a replacement window by yourself.
If you have to alter the window opening or change the exterior trim, count on a full day for each window, especially a large window. This time estimate does not include painting or staining new trim or drying time for the paint or caulk.
If you are replacing multiple windows, that 4-6 hour installation estimate likely will drop as you become more adept at the process.
Adding new-construction windows will take less time for a fully prepared rough opening: estimate around 1 to 2 hours per window. But wall rebuilding is part of the process, the window installation timeframe is dependent on the rebuilding: from one day to several days.
Determine If Replacements Will Work
If your decision to replace your own windows is based on using insert replacement windows, be aware that not all window openings are suitable for inserts.
For replacement windows to perform as designed, the existing window frame must be close to perfectly square—the diagonal measurements can vary by no more than 1/8-inch.
The frame also must be solid and have no signs of rot or structural damage. If the existing frame has a sloping sill, you'll probably need some type of sill adapter to cover the gap between the insert frame and the exterior portion of the sill. Adapters are sold separately by window manufacturers.
Before settling on inserts, check with window manufacturers to make sure that their inserts will work for your project and to confirm all installation details for the windows you have your eye on.