I think you mean rommon image instead bootstrap image, actually there is no need for you to upgrade the bootstrap image only if you are planning to run "no service password recovery" Bootstrap program some of the times references the ROM monitor, I only have seen how to upload a rommon image in IOS EXEC mode by doing the following:
820-2#copy tftp: rommon:
Address or name of remote host ? 18.104.22.168
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...