The steps are as follows:

First, download a software called "S7Edit" on the Internet, if it is a compressed package, then unzip it to be used.

Open the S7Edit program.

Then find your archive file in the game's installation directory, with a filename similar to SG7-00X.sav. Copy it to your desktop for convenience

Then open it with S7Edit, click File→Open, and then select the archive file

After you have loaded the archive, click on the generals, and then locate the one that belongs to your monarch. Here we take Ding Yuan as an example, after you find it, right click on the general's name and select Modify

Then enter the general's detailed information interface, and find the experience column in the big red box as shown in the picture below

Then left click on the numerical value column, and enter the editing state. Because the level of the general is dependent on experience, modifying the number of experience can achieve the effect of modifying the level.

After the modification, click "File" → "Save", choose a good save path, copy the new archive file to the archive directory of the game installation directory, and then enter the game again to change the level of generals.