TEMPO

To the left of the rewind button are 6 dots. Grab and pull those down.

A numerical tempo value appears in two places and changing it in each has radically different effects.

In the upper right corner of the MuseScore window is the "Playback Panel".

This shows the current tempo of the music being played. If no music is currently being played, it shows either the default 120 tempo or the last tempo played in the music. While the score is playing, you can change this as bobjp suggested: click the six dots at the left of the Playback Panel (in my screenshot, directly above the triplet button), dragging it out of the corner, and sliding the bar for a percentage change to all written tempos. Changing the percentage slider is a temporary change! This is commonly used if you want to try out different tempos "live" to find the "right" tempo for a piece.

In the score, you can enter metronome markings.

These can be changed by double-clicking the tempo marking and simply overwriting the existing number with a new one. This is a permanent change and will be saved with the file. You can include multiple tempo markings (as the screenshot shows) to change the tempo during play. A file with no tempo marking plays at the default tempo of [quarter note] = 120.

The percentage changes are a percentage of the metronome marking.

You can also set the tempo by using the Italian names for tempos, rather than numeric values. These are found in the Tempo palette. Each is set to a specific value that falls in the broad range that those Italian names signified. You can change these values by either including a metronome value (and hiding it) or by making a change on the Properties tab under Tempo. You can set it to "Follow written tempo" or to "Override written tempo".