Changing Mesh

I am trying to change a square in my mesh into a perfect circle. I have seen it done before I don’t know if it is an add-on or what. Can someone please help?

First get a mouse with a scroll wheel so you’re able to scroll down web pages. Then you’ll be able to use the support forum for your support thread rather than just post in the first one you see. Thread moved from ‘Blender & CG Discussions’ to ‘Support / Modelling’

Ensure your ‘square’ has more than 4 vertices or the ‘circle’ with be a square

The result you get will depend on your original mesh (for some bizarre reason you have not supplied any .blend file for us to look at). Select the vertices you want to change and:
Option 1 - use the Circle option in the Loop Tools addon
Option 2 - Mesh / Transform / To Sphere. Then increase the factor value

Never mind, didn’t read the question properly. My bad!