@SubSora you need to decrypt the save file first. I've written a little step-by-step guide for you and everyone who is running into this same problem.
For hex editing I would suggest using HxD https://mh-nexus.de/en/downloads.php?product=HxD but any other Hex Editor should work though.
First of all, always keep a backup of your save file!
get pbrsavetool from my post above
extract pbrsavetool.exe into the same directory where your PbrSaveData file is located
a command line window will open asking you whether you want to decrypt or encrypt your save data
put in d to decrypt
this will create two new files in the same directory, PbrSaveData_decrypted_backup (this is not an actual backup file!) and PbrSaveData_decrypted_current
open PbrSaveData_decrypted_current in your Hex Editor
get your TID and SID and convert them into hexadecimal (windows calculator has a programmers mode which can help you to do that or just use google)
example: my TID is 12345 so in hex it is 0x3039, my SID is 54321 which is 0xD431
now navigate to the above mentioned offsets for the save slot you used
how to do this depends on the programm you are using
in HxD go to Search -> Goto..., in Hex Workshop go to Edit -> Goto...
alternatively you could press Ctrl + G
make sure it's set to hex and enter the offset you want to go to
for the first slot go to offset 0x12867 and change the value at this location to the first byte of your TID (in my case it's 30)
now go to offset 0x12860 and change it to the second byte of your TID (39 in my case)
next is your SID, the first byte (D4) goes to offset 0x12866, the second one (31) goes to 0x12865
save your changes by overwriting PbrSaveData_decrypted_current
run pbrsavetool to re-encrypt your save file
this time choose e to encrypt your file and fix all the checksums
your original PbrSaveData file in this directory will be updated
PbrSaveData_decrypted_backup and PbrSaveData_decrypted_current are not needed anymore and can be deleted