function modifyCell(cellName, increase) {
var range = SpreadsheetApp.getActiveSheet().getRange(cellName);
var toAdd = -1;
if (increase) {
toAdd = 1;
}
range.setValue(range.getValue() + toAdd);
}

function increaseCounter() {
modifyCell("I2", true);
}

function decreaseCounter() {
modifyCell("I2", false);
}