It is an example of the bit of the code you would use in your code to change a attribute string value. There is more code wrapped around it. Like select a entity Blockobj and tagname the latter would be "INSTALLATION_NOTES"
Code:
(setq newstr (getstring "\nEnter new string "))
(setq blockobj (vlax-ename->vla-object (car (entsel "Pick block"))))
(foreach att (vlax-invoke (vlax-ename->vla-object blockobj) 'getattributes)
(if (= "INSTALLATION_NOTES" (strcase (vla-get-tagstring att)))
(vla-put-textstring att newstr)
)
)
It should also do a error check does Block have attributes.