Can I update iOS iPhone 6plus without wlan

Update using your computer -

Click on this link for instructions on how to update your phone using a computer

Update iOS on iPhone - Apple Support

However an iPhone 6/6 plus can't be updated beyond 12.5.7, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

To update to 12.5.7 -

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support

Connect the iPhone to a Mac or PC with an Internet connection using a cable.

If you don't have a computer, take the phone down to the coffee place or restaurant and log on to the Wi-Fi there.

Unless you have not updated the phone in a long time......you probably already have the latest update for the iPhone 6 Plus.....which is iOS 12.5.7