Reducing Function Complexity

I'm writing a simple calculator in React. I'm trying to update the state of a component based on which button is clicked (as you would expect). The component's state is as follows:

this.state = {
  total: null,
  next: null,
  operation: null,
}

Presently, I have one long if/else statement that checks a bunch of scenarios to determine what the appropriate values should be. I'm wondering how I could rewrite this to reduce the complexity of the function:

handleClick(buttonName) {
    let { total, next } = this.state;
    const { operation } = this.state;
    let obj;
    const nums = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'];
    const syms = ['+', '-', 'X', '/', '%'];
    const equal = '=';
    const reset = 'AC';
    const decimalPoint = '.';

    if (total === null && nums.indexOf(buttonName) >= 0) {
      this.setState({
        total: buttonName,
      });
    } else if (total !== null
      && buttonName === decimalPoint
      && (total.indexOf(decimalPoint) === -1)) {
      this.setState({
        total: total += buttonName,
      });
    } else if (total !== null
      && next !== null
      && buttonName === decimalPoint
      && (next.indexOf(decimalPoint) === -1)) {
      this.setState({
        next: next += buttonName,
      });
    } else if (total !== null
      && operation === null
      && syms.indexOf(buttonName) >= 0) {
      this.setState({
        operation: buttonName,
      });
    } else if (total !== null
      && operation === null
      && nums.indexOf(buttonName) >= 0) {
      this.setState({
        total: total += buttonName,
      });
    } else if (total !== null
      && operation !== null
      && next === null
      && nums.indexOf(buttonName) >= 0) {
      this.setState({
        next: buttonName,
      });
    } else if (total !== null
      && operation !== null
      && next !== null
      && nums.indexOf(buttonName) >= 0) {
      this.setState({
        next: next += buttonName,
      });
    } else if (total !== null
      && operation !== null
      && next !== null
      && ((buttonName === equal) || (buttonName === reset))) {
      obj = method.calculate(this.state, operation);
      this.setState({
        total: obj.total,
        next: obj.next,
        operation: buttonName,
      });
    }
  }

PS. the "method.calculate" function is stated in another module.