chuck said it right.
Also you can press alt hold. It is wise not to push buttons for diferent modes that the one you're in.You are level in alt hold,better push 'alt hold'. Generally,it won't do any harm to push also 'lvl chg' ,but there can be some blips in the computer,which,for ex,will start retarding,or advancing thrust,then realising it's actual mode will return to normal.Nothing dangerous but somewhat anoying.Been there,done that.
)
I usually enter the speed on the climb page
hy Alex