The tempo indicated by a metronome mark usually persists even if overwritten by an expression—such as Andante, Moderato etc. You can also add further text to a plain metronome mark. e.g.
Display the play panel: View→Play Panel or F11 (Mac: fn+F11):
Move the tempo slider up or down as required. The tempo is shown both as an absolute value and as a percentage of the currently indicated tempo mark. Double-click the tempo slider to reset it.
Note: BPM is always measured and displayed in quarter note beats per minute, regardless of the (denominator of the) time signature in effect.
You can simulate ritardando ("rit.") and accelerando ("accel.") playback by adding hidden tempo markings to the score. The printed indication to the musician should be added as staff/system text in addition.
In the example illustrated below, the tempo was originally 110 BPM (beats per minute). At the ritardando, the tempo decreases by 10 BPM on the first note of each measure. Each tempo change is made invisible by unchecking the Visible checkbox in the Inspector, so that only the ritardando shows on the printed score:
Fermatas, available in the Articulations and Ornaments palette, have a Time stretch property that can be set via the Inspector. By default, this property is set to 1.00. To have MuseScore play back a fermata for twice its normal duration, click on the fermata and set "Time stretch" to 2.00. This of course does not work for fermatas applied to barlines, as barlines don't have a duration to stretch.