MuseScore -- Index -- Tempo

Play back tempo can be changed via the play panel or via tempo text in the score.

Play panel

Tempo text
Existing tempo text can be changed by double clicking the text to enter edit mode . Beats per minute (BPM) on existing tempo text can be adjusted by right clicking on the text and selecting Tempo Properties...

Note: Tempo text in the score will override a tempo setting in the play panel.