Set odometer Value

I have a solution that works. I wanted to migrate my total distance from old computer to Garmin.

UPDATE: edited the post with working solution.  Previous one with FIT SDK was not working fully.

You'll need:
- A PC. You cannot do it on phone or via web app
- Get the fitconv tool: Releases · muktihari/fit
- Get a decent text editor, like Notepad++

Now:
- Connect the Edge to PC and once it's detected, go to folder Garmin/Totals and copy Totals.fit to the computer. Backup it somewhere in case something gets wrong
- open command prompt, cd to extracted fitconv
- call fitconv -verbose Totals.fit to create totals.csv.
Note: the verbose option is crucial here!
- open the newly created CSV file in a text editor. I wouldn't use Excel for that, to not mess something in formatting.
Example:

Type,Local Number,Message,Field 1,Value 1,Units 1,Field 2,Value 2,Units 2,Field 3,Value 3,Units 3,Field 4,Value 4,Units 4,Field 5,Value 5,Units 5,Field 6,Value 6,Units 6,Field 7,Value 7,Units 7,Field 8,Value 8,Units 8,Field 9,Value 9,Units 9
Definition,0,file_id,serial_number,1,,time_created,1,,unknown(7),1,,manufacturer,1,,product,1,,number,1,,type,1,,,,,,,
Data,0,file_id,serial_number,"REDACTED",,time_created,"4294967295",,unknown(7),"4294967295",uint32,manufacturer,"1",,garmin_product,"4061",,number,"65535",,type,"10",,,,,,,
Definition,1,file_creator,unknown(2),20,,software_version,1,,hardware_version,1,,unknown(3),1,,unknown(4),1,,,,,,,,,,,,,
Data,1,file_creator,unknown(2),"",string,software_version,"2617",,hardware_version,"255",,unknown(3),"255",enum,unknown(4),"255",enum,,,,,,,,,,,,
Definition,2,totals,timer_time,1,,distance,1,,calories,1,,unknown(10),32,,unknown(11),1,,message_index,1,,sessions,1,,sport,1,,sport_index,1,
Data,2,totals,timer_time,"ENTER_HERE",s,distance,"ENTER_HERE",m,calories,"ENTER_HERE",kcal,unknown(10),"",string,unknown(11),"0",uint32,message_index,"0",,sessions,"ENTER_HERE",,sport,"2",,sport_index,"255",
Data,2,totals,timer_time,"0",s,distance,"0",m,calories,"0",kcal,unknown(10),"KOLARSTWO STACJONARNE",string,unknown(11),"0",uint32,message_index,"1",,sessions,"0",,sport,"2",,sport_index,"255",
Data,2,totals,timer_time,"0",s,distance,"0",m,calories,"0",kcal,unknown(10),"KOLARSTWO GÓRSKIE",string,unknown(11),"0",uint32,message_index,"2",,sessions,"0",,sport,"2",,sport_index,"255",
Data,2,totals,timer_time,"ENTER_HERE",s,distance,"ENTER_HERE",m,calories,"ENTER_HERE",kcal,unknown(10),"KOLARSTWO SZOSOWE",string,unknown(11),"0",uint32,message_index,"3",,sessions,"ENTER_HERE",,sport,"2",,sport_index,"255",
Data,2,totals,timer_time,"0",s,distance,"0",m,calories,"0",kcal,unknown(10),"",string,unknown(11),"0",uint32,message_index,"4",,sessions,"0",,sport,"255",,sport_index,"255",
Data,2,totals,timer_time,"0",s,distance,"0",m,calories,"0",kcal,unknown(10),"",string,unknown(11),"0",uint32,message_index,"5",,sessions,"0",,sport,"255",,sport_index,"255",
Data,2,totals,timer_time,"0",s,distance,"0",m,calories,"0",kcal,unknown(10),"",string,unknown(11),"0",uint32,message_index,"6",,sessions,"0",,sport,"255",,sport_index,"255",
Data,2,totals,timer_time,"0",s,distance,"0",m,calories,"0",kcal,unknown(10),"",string,unknown(11),"0",uint32,message_index,"7",,sessions,"0",,sport,"255",,sport_index,"255",
Data,2,totals,timer_time,"0",s,distance,"0",m,calories,"0",kcal,unknown(10),"",string,unknown(11),"0",uint32,message_index,"8",,sessions,"0",,sport,"255",,sport_index,"255",
Data,2,totals,timer_time,"0",s,distance,"0",m,calories,"0",kcal,unknown(10),"",string,unknown(11),"0",uint32,message_index,"9",,sessions,"0",,sport,"255",,sport_index,"255",
Data,2,totals,timer_time,"0",s,distance,"0",m,calories,"0",kcal,unknown(10),"",string,unknown(11),"0",uint32,message_index,"10",,sessions,"0",,sport,"255",,sport_index,"255",

In lines 8-10 there are totals of each activity. My system is in Polish, so the names are localized. For example KOLARSTWO SZOSOWE means road cycling.
- Add your new values in fields marked by me ENTER_HERE: total time in seconds, total distance in meters, total calories, total number of activities. Repeat for other activities if needed - like MTB for you.
- in line 7, there is a grand total of all activities. Fill it now to be the sum of activities.
- save the file and pack it back to FIT format by calling:
fitconv totals.csv
- copy the new Totals.fit to Garmin/NewFiles (not to Totals!)
- disconnect the device and wait for it to load, go to History/Summary and check