Quick answer:
The guiding principle of good historic work, reflected in the Secretary of the Interior’s Standards for Rehabilitation, is to keep and repair original features (windows, trim, plaster, flooring, masonry) wherever possible.
Original wood windows, for example, can usually be repaired and weather-stripped to perform well, rather than replaced. When a feature is beyond saving, replace it “in kind”: same material, profile, and detail.
You can update systems and comfort without erasing history: discreet HVAC, updated wiring and plumbing, insulation that respects how old walls breathe.
The goal is a home that’s safe and comfortable but still unmistakably itself. Many older homes also carry the same hazards as any older property, test for lead and asbestos before disturbing original materials.

Make it safe
Handle immediate hazards first (electrical, structural, contamination).

Find the cause
Fix the source, not just the symptom, so the damage doesn't return.

Repair from the inside out
Structure and systems before surfaces.

Maintain
Protect the work you paid for.
What does "repair in kind" mean?
Can I replace my old windows?
Are there tax credits for historic restoration?
Do I need permission to renovate a historic home?