My watch slowly loses time. How can I crrect the time shown and make sure it stays there?

It should correct the time by syncing with your phone. 

time Settings
Hold B, and select > System > Time.
Time Format: Sets the device to show time in a 12-hour or a 24-hour format.
Time Source: Allows you to set the time manually or automatically based on your paired mobile device.

You can correct it manualy as well if you like, read the fine manual ;-)

My device does not display the correct time
The device updates the time and date when it syncs with your smartphone or when the device acquires GPS signals. You should sync your device to receive the correct time when you change time zones, and to update for daylight saving time.
1  Hold B, and select > System > Time.
2 Verify the Auto option is enabled.
3 Select an option:
• Confirm that your smartphone displays the correct local time, and sync your device to the smartphone (Using the Garmin Connect App, page 36).
• Start an outdoor activity, go outdoors to an open area with a clear view of the sky, and wait while the device acquires satellite signals.
The time and date are updated automatically.

I am sorry but I have tried all of those without success.

Can I come into your head office and have someone there look at it for me? If not, this will be the last Garmin product I will ever buy.

I do not work for Garmin, just a fellow consumer of their goods. I would contact Garmin suipport (on Chat) because it might be a faulty unit. I have not seen a Garmin device loose time since it syncs with the phone or GPS during outdoor activities.