WHMCS has released a new patch for WHMCS v5.3.4. They released WHMCS v5.3.5 to address few issues discovered in the previous version WHMCS v5.3.4. Since WHMCS v5.3.5 released contains bug fixes from the previous version, you should upgrade your v5.3.4 as soon as possible.
This was second update within few days from the public release of WHMCS v5.3.3. If you are using WHMCS v5.3.3 and you would like to upgrade v5.3.5. You can follow the below steps to avoid full upgrade.
 First apply the patch for WHMCS v5.3.4. It is available from the download page. You can also download it from the following URL:
 Once you apply WHMCS v5.3.4 patch, login to your admin area and verify the version.
 Now you can download WHMCS v5.3.5 patch and apply it.