Hi @Sharath2507,
A great place to start is by giving the mailbox a good clean. Begin by brushing away any loose debris, dirt, or cobwebs using a soft-bristled brush like this Oates Bannister Brush. Then wash the surface with a mixture of warm water and detergent such as this Gerni Concrete And Stone Cleaner and a scrubbing brush.
For tougher stains, a pressure cleaner would be handy, but you'd want to avoid the mortar lines, as it could contribute to cracking or damage.
Once the stone is clean, it’s worth inspecting the mortar between the stones. If there are issues, repointing the joints with fresh mortar will not only improve appearance but also strengthen the structure.
When cleaned and restored, you could seal the stone with something like this Crommelin 1L Natural Finish Penetrating Sealer And Water Repellent. This will help to bring out the natural stone look as well as protect it from the weather.
Depending on the opening in the stone, you might also be able to replace the mailbox with a new brick in mailbox.
You could also grab some new house numbers in a style of your choice.
Allow me to tag @EleventhCoastal, @Noyade and @DIYGnome to see if they have any ideas to spruce up your mailbox.
Jacob
