HTML:
<template>
<lightning-record-edit-form object-api-name="Contact" record-id={recordId}>
    
 <lightning-checkbox-group name="Checkbox Group"
                              label="Checkbox Group"
                              options={options}
                              value={value}
                              onchange={handleChange}
                              onload={handleOnLoad}>
                              </lightning-checkbox-group>
<lightning-input-field class="slds-hide" field-name={selectedValues} value = {selectedValue} ></lightning-input-field>
    <div class="slds-var-m-top_medium">
        <lightning-button variant="brand" type="submit" label="Save">
        </lightning-button>
    </div>
</lightning-record-edit-form>
</template>

JS:
import { LightningElement,api,track } from 'lwc';
import TITLE_FIELD from '@salesforce/schema/Contact.Title';
export default class RecordEditFormLWC extends LightningElement {
// Expose a field to make it available in the template
    selectedValues = TITLE_FIELD;
@track isChanged =false;
@track value ;
@track titleold;
    handleOnLoad(event) {
    var record = event.detail.records;
    var fields = record[this.recordId].fields; 
 /*   const titleNew=fields.Title.value;
    this.titleold=titleNew;  */
     
    if(fields.Title==="option1"){
    this.value.checked="checked";           
    this.isChanged=true;
}
}
    // Flexipage provides recordId and objectApiName
    @api recordId;
    @api objectApiName;
    value = [];
    get options() {
        return [
            { label: 'Ross', value: 'option1' },
            { label: 'Rachel', value: 'option2' },
        ];
    }
    get selectedValue() {
        return this.value.join(',');
    }
    handleChange(e) {
        this.value = e.detail.value;
    }
}