Unable to change the speed

Not sure what you'd want a "display" button for - are you perhaps looking at documentation for a much odler version of MuseScore? For the current documentation, see the Handbook - either via the Help menu within MuseScore or in the menu at the righthand side of this page. The speed at which a piece of music plays is called the "tempo", so that is what you should be looking up in the Handbook. Short answer: to place a tempo marking in the score (which is how you tell a performer - whether human or computer - what tempo you want), click the note where you want it to appear and press Alt+T. You can then enter your tempo marking.

If on the other hand you want to temporarily override the actual tempo for the piece - like to play back more slowly for practice purposes - then you want the Play Panel, which in current versions of MuseScore is found in the View menu.