how can we increase gain of colplanar patch antenna

Hi

There are many ways to do that... It depends on the fabrication facility you have.
The simplest is to make antenna array, other than that you can use periodic structures in substrate (meta material) to increase the directivity. There are many papers on what so called superstate and how it increases the gain

cheers,

As your coplanar patch antenna is a small radiator similar to a dipole, you can increase its gain by 3 dB or more by using a reflector element, reflective backing plane or even a parabolic dish.
Reflectors generally limit the radiation pattern by turning back one hemisphere into your antenna radiates. The radiation pattern of the antenna with a reflector will narrow on one or both planes- this increases directivity and gain.

There are other ways like making an array of patches, etc., but it will need to make more radiators and create a phase-referenced feed network which is quite a lot of work. Try a reflector!