You want to edit the RACI template, e.g. you want to change the column size or name, add a new column and so on.
In Visio open Template (.vst) file (e.g. Raci (Hoch) - BP.vst), switch to the background page, select a shape (for example, “R”) and edit it. Please note, that Shapes, Input, Activity, Output and others are protected. On the Developer tab click Protection button. Deactivate all fields in the opened window.
Then On the Developer tab click Behavior button. The settings should look like this.
Now you can edit the shape. Afterwards it should be again protected so that it cannot be freely moved on diagrams. Protection settings should look like this:
Save the stencil.
If then you create a new diagram, it will be created with the edited stencil.