I don't know how to save this tree. The top half seems completely dead, the lower half not too bad. Should I just saw off the top half?
What little I can see looks like Norway spruce, aka Christmas tree. It won't sprout from old wood and will never look any good, so get rid. You can get another in a couple of months if you really want, but it is never going to be a long term option. They are forest trees growing to over 60ft and unsuited to growing in a pot.
Thanks, but I am not concerned whether it will look good.
I want to know whether it has any chance of survival and how to maximise its chances.
It may be alive where it's green, or it may be dead but just not showing yet. Otherwise it's very dead indeed. Conifers do not recover from brown leaves (or no leaves), What you have there is the best you will get, downhill from now.
Totally agree with the others ... but if it has sentimental value and you feel you must do something then cut off the stem where the brown branches start and repot it in JI No 2 ... but I'm afraid it'll be a waste of time and money.
Many Guy Fawkes bonfires contain last year's Christmas trees which have been kept unsuccessfully over the summer.