Currently one of my tires botak on the outer side due to uneven wear. That tyre also has some tear on it.
Can i just change that tyre only or have to change a pair? If i change a pair, what can i do with the other tyre? Other tyres are left with about 40% thread.

u can, but you shouldn't

the "correct" answer is:
a 100% new tyre vs 40% is going to have significant difference in performance. run over a puddle of standing water and you will know it in a very painful way.

just change them in pairs and consider the other tyres a lost cause (thou the tyre shop should be willing to take it in for a few bucks)

can, but not recommended. Is like changing shoe. Would you like to change a shoe or a pair?

How about your spare, is it the same size? If not, then better buy a pair and then make the used 40% one as spare tire. Cuz the difference is just too much if the other one only left 40%. If still like 70-80% then still can.

The spare tyre is not the same size. Will the tyre shop pay something to me for the good 40% tyre or i better take it home just in case i need it in the future?

Some of my customer will get one tyre each time they got their salary. If can, best is change a pair.

Used tire with only 40% left will not get you anything much. Imagine that even tires that are still 80-90% condition are only priced around 25-35% of the new price, so yours being only 40% left probably can only fetch like 10% of the new price, that's IF they want to take it also! But you can try to ask them.

I suggest you buy a pair and put the used one as spare if you want.