Sometimes you need to execute code before submitting the form and set or modify some fields on sObject.

For example, you have the following lightning-record-edit-form for Account sObject.

<lightning-record-edit-form object-api-name="Account" 
                            onsubmit={onSubmitHandler>
<div class="slds-grid slds-wrap slds-gutters">
    <div class="slds-col slds-size_1-of-2">
        <lightning-input-field field-name="Name" variant="label-stacked"></lightning-input-field>
    </div>
    <div class="slds-col slds-size_1-of-2">
        <lightning-input-field field-name="Phone" variant="label-stacked"></lightning-input-field>
    </div>
</div>
</lightning-record-edit-form>

To modify some fields you can use onsubmit handler. You can get created/updated sObject from the event object. After the modifications, you need to submit the form.

onSubmitHandler(event) {
    event.preventDefault();
    // Get data from submitted form
    const fields = event.detail.fields;
    // Here you can execute any logic before submit
    // and set or modify existing fields
    fields.Name = fields.Name + fields.Phone
    // You need to submit the form after modifications
    this.template
        .querySelector('lightning-record-edit-form').submit(fields);
}

I and our TrueSalesSoft team can help you with any questions related to Salesforce custom development. You can use Contact page on truesalessoft.com.