Where is the variable stored? Callback functions take at least 2 inputs, always. Here is a simple demo, illustrating one way to do what you want. Note that I have made it step by step so you can take the semicolons off to see what is going on in the callback if needed.
function [] = demo_cb()
S.fh = figure('units','pixels',...
'position',[300 300 300 130],...
'menubar','none',...
'name','demo_cb',...
'numbertitle','off',...
'resize','off');
S.pb = uicontrol('style','push',...
'unit','pix',...
'position',[10 20 280 90],...
'string','I=1',...
'fontsize',14,...
'callback',@pb_call);
function [] = pb_call(H,E)
% Callback for pushbutton.
Str = get(H,'string');
Num = regexp(Str,'=','split');
NewNum = str2double(Num{2})+1;
set(H,'string',sprintf('I=%i',NewNum))
