make sure honda approves MT-90 fluid.
some require SPECIFIC fluid some require general fluid.
my Jeep needed "jeep specific" manual tranny fluid and or a GM synchromesh or it destroys synchros
so do your studying before you do anything
BUT it is never early to change fluids. Keeping it fresh will only be beneficial
and as far as the drain plug goes, it should be in the manual. I don't have a manual but i'm willing to bet it's on the driver side. two bolts. fill and drain. DO NOT OVER FILL