Editing keychain. I can't sync my Garmin because my apple keychain seems to have the wrong password. How can I edit keychain?

It sounds like there is some disconnect but I'll be darned if I know what that is. It's quite odd for an app to report an incorrect password before the password has been entered, unless there is an automatic login which fetches the password from the keychain. It's even possible there is a corrupted preference or cache file involved.

Here's is what I suggest you try. It will help determine if the problem stems from your user account.

A Troubleshooting Procedure that may Fix Problems with macOS El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

1. Shutdown the computer and disconnect all third-party peripherals.
2. Wait 30 seconds before you restart the computer.
3. Resetting your Mac's PRAM and NVRAM
4. Reset the System Management Controller (SMC)
5. Reset your Startup Disk and Sound preferences, if needed, after resetting the PRAM.
6. Test in safe mode to see if the problem persists, then restart normally. See How  to  use  safe  mode  on  your  Mac.
7. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list.  Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
8. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.

If the problem disappears between Step 1 and Step 7, then there is no need to do Step 8. If you do Step 8, and then the problem is fixed, that means the problem is with something in your user account.

What warning or alert do you receive that requests a different password. Is this what is happening: If your Mac keeps asking for the login keychain password?

Keychain editing is done vie Keychain Access in the Utilities folder. See An essential Mac OS X Keychain guide and Everything You Need to Know About Keychain in macOS Sierra.

Repair Keychain in El Capitan and Later

1. Launch Keychain Access (located in Applications/Utilities)
2. Click "Show Keychains" in the lower-left corner of the window.
3. Select the problematic keychain from the left-hand pane.
4. Navigate to the "File" menu and select "Delete Keychain '(name of keychain)'"
5. Check all options for deletion and press "OK"
6. Create a new keychain by going to the "File" menu, then "New" and selecting "New Keychain"
7. You can now make this keychain your default if you desire by selecting it, then  going to the "File" menu and selecting "Make '(name of keychain)' Default"