How can I download iOS 13.6 without using updates in my settings

• If you have an iPhone 6 or older, the highest you can update to is 12.5.3

• If you have an iPhone 6S or newer, you can update to 14.5.1

Update your iPhone, iPad, or iPod touch -

https://support.apple.com/en-us/HT204204

The only way to download iOS is through the "Update" section in Settings. As long as your phone can support it, it's going to download the most recent version of iOS. If it cannot, then it is going to download the highest version that it is capable of running. It is not possible to download an older version of iOS if your phone can support the current version that is out because once an update is released that your phone can support, that is the version that is going to download when you do an update.