Welcome to the community!
You should be able to make the purchase online.
Just login to your account select the number you want to make the purchase on then select UPGRADE DEVICE, chose the phone you want then select Keep existing plan and then check out.
You can also make the purchase at any Fido retail store but they might want to charge you a $35 fee.
Depending on the phone you want it might also be better to buy it directly from the manufacture, the only down side to doing this it's not guarantee that all the features might work like VoLTE and WiFi calling.