Hi,
Ok and Cancel buttons are hardcoded in inputdlg. You can copy the code of inputdlg.m and make changes in the CancelHandle String as shown in below code snippet.
CancelHandle=uicontrol(InputFig , ...
BtnInfo , ...
'Position' ,[ FigWidth-BtnWidth-DefOffset DefOffset BtnWidth BtnHeight ] , ...
'KeyPressFcn',@doControlKeyPress , ...
'String' ,'More Options', ... %getString(message('MATLAB:uistring:popupdialogs:Cancel')) , ...
'Callback' ,@doCallback , ...
'Tag' ,'Cancel' , ...
'UserData' ,'Cancel' ...
);