Firstly, you will need to download a suitable software image from CCO. You will need a CCO login with appropriate privileges to do so. Verify that you have sufficient Flash and DRAM for the image you are downloading.
Then, follow the procedure in the following document in order to actually upgrade the code:
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...