1. First, open the target PCB file with protel DXP software, and then set the layer as "occlusion" layer (no wiring layer).
2. Then use the routing tool to draw a new size for the PCB size, as shown in the figure.
3. Then press and hold the "shift" key to select all the external borders you just drew.
4. On the Design menu, click the option "Plate shape/defined by selected object", and then click Yes in the dialog box that appears.
5. After the above settings are completed, the size of pcb can be changed, but it needs to be covered with copper again.