A. This patch file are from the core devs.
B. You must replace the file with the patch file.
How to Backup an Installation and Upload the File:
1. Make a backup of your installation. Please refer the how-to backup using Softaculous here.
2. Download the patch file (1 file) to your PC fast_checkout.zip
3. Extract the file if you want to upload via FTP client.
4. There are two ways to upload the file:
a. Login to cPanel, click on public_html/your_site/, upload the zipped patch file and extract. Refer to our how-to here or
b. Open your FTP Client, click on public_html/your_site/, upload the extracted file there.