How to make place a block

I want to make if floor is glass, create a sand above to glass(y+1)

 

If i understand correctly, you want to place a sand block if the block under it is glass?
If you're talking about doing that on an area, once, i would suggest using a plugin like Builder Tools, or WE
If you you want to do it under a player, You can listen to play movement events, check the block under the player, and then use setBlock() to create the sand block. Please repsond with more info so that we can help you