Sometimes you just want to start over and work with a blank canvas. To do that type the following.
Sometimes you want to target a specific turtle or a group of patches, to do this we can use the following commands.
ask turtle 0 [ set shape "airplane" ]
ask link 0 1 [ set color brown]
ask patch 0 1 [ set pcolor orange ]
Targeting a group of links
ask turtles with [color = red] [set color orange]
ask links with [color = red] [set color blue]
ask patches with [pxcor > 0] [ set pcolor yellow ]
ask turtles with [(xcor > 0) and (ycor > 0) and (pcolor = green)] [ set color brown ]
ask patches at-points [[1 0] [0 1] [-1 0] [0 -1]]