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It depends on what technology you're using. We would need more info 🙂

Manually, you should probably be able to separate corner pieces from each of sides pieces .. imagine 3 x3 grid and you cut it up into 9 pieces with 4 corners, and then each 4 sides and a center.

In Godot use NinePatchRect and you input the # of pixels each corner is.

in Unity, you add Sprites package and setup a 9-slice sprite .. see youtube for tutorial ... 

Idk other game engines right now...