Default BPM

The BPM text entry in the upper left corner of the editor, or in the bottom bar, lets you set the default tempo.
Global BPM in the editor

BPM text

BPM track

The BPM track lets you override the default tempo for a block.

To enable the BPM track, right-click somewhere in the timing area of the editor (to the left of the note tracks), and enable "BPM".

 BPM format: xxxt, where
    
 xxx  = BPM. 1 = lowest BPM, 999 = highest BPM.
    t = Whether to gradually change tempo between the current BPM and the next (denoted by '|').

 If t is ' ' (empty), the tempo will accelerate / deaccelerate to the next BPM.
 If t is '|', there is no accelerando / deaccelerando.

 Press T to switch between the two modes.
      
 Notes:
  * If xxxt has an 'm' prefix, it means that the line contains more than one BPM entry. The last BPM is displayed.
  * If xxxt has a 'd' prefix, it means that the BPM entry starts below the start of the line.

Tips:

To remove a BPM entry, press DEL.
To adjust accelerando and deaccelerando behavior (relevant when t is empty), go to File -> Song properties.