Hello Aelsharawi,
Unfortunately you can't switch fields because each fields has different settings. You can just drag / drop a new field into the field form
By the way, I am not too sure if I understand you about multi select with checkbox? Multi select is basically a dropdown list that you can select multiple values from it. In other words, it's a dropdown that allows you to select multiple values. Not really sure how "checkbox" would work in this though?