Hi,
I just bought a treadmill and it says it needs to be on a 20 amp breaker.
Is it proper to just switch the 15 amp for a 20 amp? Or do I need to upgrade the wiring as well?

The minimum wire size for a 20 amp breaker is #12 wire, so, no, unless the existing wire is #12, then you cannot just replace the breaker.

Perhaps you can have a separate 20 amp circuit and receptacle installed just for the treadmill.

Thanks for the reply, not the answer I wanted really.lol
I guess I'll have to upgrade that line or run a new one.

Sorry, not here to be popular, just to give the best accurate answer that will protect lives and property.

Thanks for the reply, not the answer I wanted really.lol
I guess I'll have to upgrade that line or run a new one.Don't bother "upgrading" the circuit. Just run a new line to the treadmill.