Drain plug - 3/8" square drive
If you have been changing your oil regularly then you only need to do a simple drain and refill. If your oil is discoloured - It should be a shade of red - or if you have not changed it for 60,000km then you should do a complete change and replace the transmission oil filter.
The Colour of transmission oil - How do you know it's burnt?

This has been a bone of contention to me for a while. New ATF is red but it can quickly loose that bright colour. The colour of ATF, unless it's almost black, is not an indication as to the usefulness of the fluid. The consistency and the smell are the key factors to look for however, unless you have smelt burnt ATF how do you know what it smells like? Compare it to new fluid if unsure.

If you can smell the trans dip stick of a friend's car that has less than 20,000km on the fluid then this, I think, is a better indication of what the fluid on your dipstick should smell and look like when the dip sticks are wiped and compared on a tissue. His car need not be a Honda or use Z1.

People talk about fluid being black; I assume that they are talking about the oil on the dip stick. New ATF-Z1 when placed in a glass jar can look black not red.

The fluid that you drain into a deep pan will look very dark - don't fall for this illusion. Place a drop on a clean tissue and then compare the colour to new fluid.

Even if the ATF is a shade of red it should still be replaced, do it every 20,000km to 30,000km to flush out contaminants lying at the bottom of the transmission housing and to remove the build up of metal on the magnetic drain plug.
Note: Warning you follow these suggestions at your own risk.

When draining the oil, leave the car level. If you jack the car up at the front you will not drain the transmission correctly.

Find the drain bolt, it's low on the front left hand side of the transmission; you want that to be at the lowest point to correctly drain the oil and contaminants from the pan.

If the ATF filler bolt is not occupied by a 2nd gear oil jet modification then you have the option of refilling the transmission through the ATF filler hole.

I decided not to do this because:

1. The filler bolt is very - very hard to remove.
2. I found it a pain to refill without spilling fluid with the filler plug down so low.
3. A refill jig used to fill through the dip stick tube is so easy, it just takes time. Have a hit of your fav addiction while waiting.

A simple drain and refill requires about 3 litres of fluid.

1. Drive the car until it is at operating temperature, check that the fluid level is correct - Correct it now or adjust the refill amount in step 4 if it's incorrect.

2. Remove the drain plug and drain the fluid - measure how much fluid you removed.

3. Fit the drain plug using the new washer and tighten - do not over tighten. 49 N-m (5.0 kgf-m, 36 lbf-ft)

4. Replace the amount of fluid you drained with new trans fluid. - Replace with the same amount you removed or Adjust for step 1.

5. Start the car, check for leaks then take the car for a drive, get it to change through all gears, get it to operating temperature.

6. Turn the car off and check within 60 seconds for the correct transmission fluid level.

7. Add or remove fluid to bring the fluid to the correct level.

Only measure the fluid when the car is absolutely level - Not on a slope.

The transmission fluid must be measured with the engine at operating temperature and within 60 seconds of shutting the engine down. Check your manual to confirm just in case.

Never measure after a high speed run or when the engine/transmission is overly hot.

Once you know that the oil is at the correct level, you can measure it when it's cold and note the reading on the dip stick, it may be different than when the transmission is hot.This reading can be used to check, before you drive, that the correct amount of oil is most likely in the transmission.

Still do regular checks at operating temperature though.