You must engage a Building and Construction Authority (BCA) approved window contractor listed in HDB's directory for all window installation/ replacement work. These are the requirements that you need to comply with for window installations.
As a flat owner or occupant, you are responsible for the upkeep of the windows in your flat. Otherwise, a window could fall and cause serious injury or worse to those below.
Guidelines on other windows and grilles works.
The replacement of full height windows is strictly not allowed. Permission may be granted in an instance where a damaged frame requires a complete replacement.