updateRecord from the ‘lightning/uiRecordApi’ library can be used to update records without using Apex in Salesforce Lightning Web Component.

Sample Lightning Web Component:

HTML:

<template>
    <lightning-card icon-name="standard:case">
        <p class="slds-p-horizontal_small">
            <lightning-input 
                type="text"
                label="Subject"
                placeholder="Enter new Subject"
                onchange={handleInputChange}
            ></lightning-input>
            <lightning-input 
                type="text"
                label="Description"
                placeholder="Enter new Description"
                onchange={handleInputChange}
            ></lightning-input>
            <lightning-input 
                type="text"
                label="Comments"
                placeholder="Enter Case Comment"
                onchange={handleInputChange}
            ></lightning-input>
        </p>
        <p slot="footer">
            <lightning-button
                label="Update Case"
                onclick={updateCaseRecord}
                variant="brand"
                icon-name="standard:record_update"
            ></lightning-button>
        </p>
    </lightning-card>
</template>

JavaScript:

import { LightningElement, api } from 'lwc';
import { updateRecord } from 'lightning/uiRecordApi';
import { ShowToastEvent } from 'lightning/platformShowToastEvent';
import { RefreshEvent } from "lightning/refresh";
import DESCRIPTION_FIELD from '@salesforce/schema/Case.Description';
import SUBJECT_FIELD from '@salesforce/schema/Case.Subject';
import COMMENTS_FIELD from '@salesforce/schema/Case.Comments';
import ID_FIELD from '@salesforce/schema/Case.Id';

export default class QuickCaseUpdate extends LightningElement {

    @api recordId;
    subject;
    description;
    comments;

    handleInputChange( event ) {

        const label = event.target.label;

        if ( label == 'Subject' ) {

            this.subject = event.detail.value;

        }      

        if ( label == 'Description' ) {

            this.description = event.detail.value;

        }      

        if ( label == 'Comments' ) {

            this.comments = event.detail.value;

        }        

    }
    
    updateCaseRecord() {

        const fields = {};        

        if ( 
            this.description && 
            this.description.trim().length > 0 
        ) {
            
            fields[ DESCRIPTION_FIELD.fieldApiName] = this.description;

        }

        if ( 
            this.subject && 
            this.subject.trim().length > 0 
        ) {
            
            fields[ SUBJECT_FIELD.fieldApiName] = this.subject;

        }

        if ( 
            this.comments && 
            this.comments.trim().length > 0 
        ) {
            
            fields[ COMMENTS_FIELD.fieldApiName] = this.comments;

        }

        if ( 
            Object.keys( fields ).length > 0 
        ) {

            fields[ ID_FIELD.fieldApiName ] = this.recordId;

            const recordInput = { fields };

            updateRecord( recordInput )
            .then( () => {

                this.dispatchEvent(
                    new RefreshEvent()
                );
                this.dispatchEvent(
                    new ShowToastEvent( {
                        title: 'Success',
                        message: 'Case updated',
                        variant: 'success'
                    } )
                );

            } ).catch( error => {

                this.dispatchEvent(
                    new ShowToastEvent( {
                        title: 'Error updating Case',
                        message: error.body.message,
                        variant: 'error'
                    } )
                );

            } );

        } else {

            this.dispatchEvent(
                new ShowToastEvent( {
                    title: 'Error updating Case',
                    message: 'Please enter Case details',
                    variant: 'error'
                } )
            );

        }

    }
}

js-meta.xml:

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>61.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__RecordPage</target>
    </targets>
</LightningComponentBundle>

Output: