That is a strange looking tree.  It seems to have two upright branches that are maybe tied together and no side branches.  Also, what is tied on the trunk?  What is the soil like??  Is it sandy or clay or good soil?  How was the planting hole prepared?  Did you dig a hole that was wider than deep.  Your tree is probably grafted to a dwarf rootstock.  That graft should be above ground. Not buried.  Finally, how much and often are you watering?  Get back to me with responses and we will try to see what is happening.

I am still waiting for a response to my questions.  I cannot give you an answer without more information.

The soil is clay.  We dug a wide hole as per instructions. The depth is as deep the pot it came in. Because we replanted from pot so I have watering every other day.

The bottom part has branches and leaved that are healthy. We have tied it for support.

Untie those branches.  Tree branches are supposed to go outwards.  It is possible, because you soil is clay and you did not add any compost or loosen the soil in the hole prior to planting, that the water is not draining as it should be and the roots are starting to rot.  You need to dig down and see what condition the roots are in.  They should be firm and white.  If they a mushy or turning black, they are rotting.  Dig your tree out, loosen the soil at the bottom of the area and mix in some compost.  Then cut off any rotting roots and plant the tree high in the hole.  That will allow the roots to get some oxygen and keep them up and away from standing water.  Good luck.