The router will show a progress bar. The device will reboot automatically once finished. Key Improvements in V6.20 Firmware
Turbocharge Your TP-Link TL-WR840N (ME) V6.20 : The Ultimate Firmware Guide tlwr840nme v620 firmware full
If you need a specific older build (like the 201124 version), you can sometimes find verified dumps on Internet Archive Step 3: The Safe Flashing Process Download for TL-WR840N V6.20 - TP-Link The router will show a progress bar