So you want to change the tempo.
You can accelerate (speed up) or decelerate (slow down).
Acceleration comes naturally to quickening notes, like this passage whch moves from minims to quavers. Use your notes to assist the acceleration process.
Do not accelerate a passage moving from quavers to minims.
Write accelerando or accel. under the measures you want
Deceleration comes naturally to slowing notes (minim to semibreve).
Write ritardando or rit under the measures you want to
Say the music begins at crotchet equals 120. You slow down to 100. You want to accelerate the tempo back to 120 (the original tempo).
This music accelerates back to the original tempo. Presumably there has been a prior deceleration. A Tempo means original tempo.
To accelerate to a specific tempo, write the metronomic figure over the desired spot. The composer wants the first two bars to accelerate to 120.