Here’s the most relevant entry I found related to editing silkscreen text:
"To add a silkscreen element:
- Left-click on the Layout object in the objects tree to ensure it's selected
- Right-click > Add > Silk Line / Silk Circle / Silk Rectangle
A silk text is a particular kind of silkscreen that lets you add text to your PCB.
A silk line is a line that stretches between two specified points. You can use a line path rule for any desired curves.
A silk circle is a circle with a specified origin and diameter.
A silk rectangle stretches between two specified points.
The stroke length, spacing, style, width, and width of these three silk elements can all be modified using the appropriate rules.
Modifying Existing Silkscreen"
Source