The warning comes on when the signal is weak.. Not sure it can be quantified as just 6 warnings. The key will continue to make transmissions, further weakening the battery even if you do not use the car key. So if you leave it too long before replacing the battery its possible the battery will be completely flat next time you try to use the car,with no second warning, let alone 6. If temperature rises the battery may again preform just well enough to stop the warnings for a while. It doesnt mean everything is now ok .The battery may also fail suddenly as temperature drops. You have been warned and its best to replace the battery asap.
Another possibility is the weak signal was because the key was shielded, maybe in a bag etc ,rather than a weak battery. It may now be in stronger signal area .
Changing the battery is easy. First remove the 'emergency key' from the remote . (BTW this key will get you into the car if the battery is completely flat) . This reveals a location where you can put a coin, or large flat screwdriver between two prominent blocks and by twisting prise the top and bottom halves apart. Its quite robustly made so it does take quite a bit of force the first time. Not beyond physical ability, but I was starting to wonder if I was doing it wrong or might damage the key before it finally unclipped (maybe use a screwdriver whilst conveniently at home , but a coin or even the removed emergency key will do the job) Once apart replacing the battery is obvious and easy and it clips back together with a reassuring quality feel . The instructions advise wrapping the coin in a cloth to prevent scratching the remote. I no longer bother. It also advises care to avoid losing the buttons, which might put you off. IMO the buttons seem secure with little risk. See photo.
Dont be tempted to use budget priced batteries. These may only last a few weeks at best. A few brands have a security insulating sticker that must be removed before use but may not be obvious.
I have always had to pay extra for an mot at any previous dealerships (of all brands) and my current Honda one. If you have several Honda dealerships nearby (increasingly uncommon) maybe ring round and ask. Even if they normally charge for the MOT they might offer it free to secure your custom ,especially if you can mention going to a rival dealership who actually do free MOT's. ( but how far extra is it worth travelling to save the fee? )