Do you mean the typical fixed window in your entry door.....inside/outside frame with glass between......

The inside frame comes off (screws)
Glass is set in outside frame using sealant and sometimes small blocks on bottom
If sealant dries out and/or blocks slip the glass will slide down
When removing inside frame have a helper outside to hold frame glass so it doesn't fall out
Clean up, install new sealant to hold glass in place...new butyl tape around inside & outside frame.