I recommend to do a Backup before an Upgrade, here its explained how u can upgrade fastforwarrd. For more information checkout: https://www.debian.org/releases/trixie/release-notes/upgrading.en.html
https://www.debian.org/releases/trixie/release-notes/issues.en.html#ch-information

1.Update Repository Sources
You must point your system to the new Trixie repositories by replacing all instances of bookworm with trixie in your configuration files.

Main sources File:

sudo sed -i 's/bookworm/trixie/g' /etc/apt/sources.list

Additional repository files (if any):

sudo find /etc/apt/sources.list.d -name "*.list" -exec sed -i 's/bookworm/trixie/g' {} +

2. Perform the Upgrade

The upgrade is best performed in two stages to minimize dependency conflicts. 

Step A: Minimal Upgrade
Download the new package lists and perform a safe upgrade that does not remove existing package.

sudo apt update
sudo apt upgrade --without-new-pkgs -y

Step B: Full Upgrade
Complete the process by allowing apt to install new dependencies and remove obsolete ones.

sudo apt update
sudo apt full-upgrade -y

3.Finalize and Verify
Cleanup: Remove old, unnecessary packages.

sudo apt autoremove && sudo apt clean

Verify Version: Check that your system reports version 13.

cat /etc/debian_version
  or 
lsb_release -a