set-text-properties

set-text-properties is a built-in function in `C source code'.

(set-text-properties START END PROPERTIES &optional OBJECT)

Completely replace properties of text from START to END.
The third argument PROPERTIES is the new property list.
If the optional fourth argument OBJECT is a buffer (or nil, which means
the current buffer), START and END are buffer positions (integers or
markers). If OBJECT is a string, START and END are 0-based indices into it.
If PROPERTIES is nil, the effect is to remove all properties from
the designated part of OBJECT.