Piano at bottom of screen

I do not completely get what you mean but I'll give it a shot.
You can open the instruments menu by pressing 'I'. To arrange the order of the instruments, click the instrument you want to move on the right, and click the arrow keys on the middle to move it. To arrange the order of the staves, click the stave and click the arrow keys.
If you want to leave a blank space above your piano staves, you can add a vertical frame above it. Click on the first measure on each page, go to palettes -> frames and bars -> insert vertical frame with the up and down arrow in dotted square. Adjust the frame height in the inspector.
Hope you find this helpful!

However, if you mean the virtual keyboard used for inputting notes onto the score then just grab the bar in which the words "piano keyboard" are printed and drag it into position

To be clear: the default position of the on-screen piano keyboard that appears when you press "P" or use View / Piano Keyboard is docked to the bottom of the window. If you are seeing it somewhere else, that would be because you have previously moved it, and MuseScore remembers the position. If you wish it to appear somewhere other than where it currently is, simply drag it. If you wish to return it to its original position docked at the bottom of the window, be sure to drag it all the way to the docking station, otherwise it will cover the bottom of your score.

Recently had this question and solved it.
If you want to move the virtual piano wherever, left lick+drag the bar containing "Piano Keyboard" at the top of the virtual piano window.
If you want to have it snap to the bottom of the window, double click the bar