The cheapest and most DIY way is to add R30 insulation in the attic.
Replacement double pained windows.
Far more expencive and time consuming you can add straping on the inside and or outside and up to 1-1/2 foam on the inside and or outside, add sheetrock over the straping on the inside and add jamb extentions to the window and door opening, on the outside you would have to build out all the window and door trim wrap the trim with aluminum coil stock then add some form of siding.
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