To add a new graph, delete one, or resize it:  click ‘Edit’ in the top right corner of the Dashboard.  This will bring up new options as shown below.

To remove a graph:  Click the small bin (or trash can) in the upper left corner of the graph.

To add a graph:  Choose a tile from the selection on the left to add a KPI or chart.  Drag the tile into the Dashboard, drag it to the new location and release.  Select a chart and edit as required. Then exit  An overview of this can be found in Getting started with Dashboards .

To move a graph: Click the top bar of the graph, drag it to the new location and release.

To resize a graph: Hover over the lower right corner of the graph until you see a double ended arrow, then drag to the desired size and release. 

Note that there is a minimum graph size, so you can sometimes only make a graph larger, not smaller.