How to change the predefined SMS on Android ?

Unfortunately, it's not possible. I am perplexed why Garmin omits basic features (while adding useless fluff) that most of us have had on our various devices for quite a while. Many here have asked Garmin to include this feature in an update.

In realtà si potrebbe fare modificando con il pc il file text all'interno della cartella garmin ,trovare la frase che si vuole cambiare e modificarla a piacere,non si può farlo dal dispositivo.

i finded it on PC

Garmin\Text\Polish.gtt

open on notepad or wordpad

section
TXT_CANNED_MESSAGE_List_Busy_At_The_Moment_STR_M
TXT_CANNED_MESSAGE_List_Disregard_Last_Message_STR_M
TXT_CANNED_MESSAGE_List_Be_Home_Soon_STR_M
etc

thx

Thanks I was able to correct spelling mistakes :-)

each message is saved in two places in the same file

each of them needs to be changed

1 is responsible for displaying the message
2 is responsible for the content of the text message

ça marche parfaitement bien

exemple : 

<str>

        <tag>TXT_CANNED_MESSAGE_List_Out_Riding_STR_M</tag>

        <txt>Sur mon velo SMS SVP</txt>

    </str>

        <tag>TXT_CANNED_MESSAGE_Prompt_Out_Riding_STR_M</tag>

        <txt>Sur mon velo SMS SVP</txt>

En mettant le même texte sur list et prompt, on obtient un résult parfait

ATTENTION : PAS D'ACCENT DANS LE TEXTE