In assets\minecraft\models\items there is shield.json. Open it with a text editor and find

 "firstperson_lefthand": { "rotation": [ 0, 180, 5 ], "translation": [ 10, 0, -10 ], "scale": [ 1.25, 1.25, 1.25 ] 

Edit the middle number in "translation" from 0 to a negative value to lower the shield.

Lower shield with value -3

Default with default 0

Alternatively you can use this and place it in the resourcepack folder like any other resource pack. Make sure to place it above other texture packs so that it will overwrite the shield file.