>> surf(peaks)
>> ticklabels = get(gca,'XTickLabel');
>> newTicklabels = [ticklabels(2:end,:);'60'];
>> set(gca,'XTickLabel',newTicklabels);
Note that ticklabels are returned by the second line (above) is a 2 column char matrix. It can also be specified as a cell array of char (strings) which is often more convenient.
>> set(gca,'XTickLabel',{'1','14','341','123412','1432','1'}); % "random" labels