Acceleration is measured in meters per second per second: m/s/s or m/s².
This means we need to find out how many m/s are travelled every s.
Which is the same as change in velocity over time.
We can work this out by looking at a time period in the graph and seeing how much the velocity changes. For example on this graph between second 3 and second 4 the velocity changes from 0 to 4: meaning that it changes 4 m/s every 1s
So effectively you do up divided by across to give acceleration.(bbc)