Vue.js back to default select option value

I have a form that using combobox. Here's my view html

<div class="form-group" v-if="onDemandLeave == true">
  <div class="row">
    <div class="col-md-2">
      <label class="col-form-label">On Demand Leave Type</label>
    <div class="col-md-10">
      <select name="LeaveType" @change="setAvailableLeave" v-model="leaveType" class="form-control">
        <option disabled :value="null">Select One</option>
        <option v-for="(type) in onDemandLeaveType" :value="">

and here's my script

onLeaveTypeChange(this) {
    if (this.selectedLeaveType == 1) {
        this.onDemandLeave = false;
        this.availableLeave = this.leaveState.leaveQuota;
    } else {
        this.onDemandLeave = true;
        this.availableLeave = 0;

so I want i have another combobox having 2 value, let's say A dan B if i choose B i want to show LeaveType combobox, if i choose A, i want to hide it

since it is a reactive page, i face a problem, if I choose B then select some value in my LeaveType combobox, then back to A, then back to B again, the LeaveType combobox still on my last selected value

can i reset it to default value using @change method?