You can edit task dependencies from the table portion of Gantt chart.
Locate the task that you want to edit the dependency for in the table portion of the Gantt chart.
Double-click the Predecessors column in a task's row. If you don't see it, click the gear icon in the table's upper left-hand corner and check the box next to Predecessors.
Replace the text in the column with the row number of the task that should be the predecessor task or change the type of dependency by replacing the letters that follow the row number:
FS – to create a finish to start dependency
SS – to create a start to start dependency
FF – to create a finish to finish dependency
SF – to create a start to finish dependency
If you only enter a number then a finish to start dependency is created by default.