Step 1: Download and copy the latest IOS (XE) to the 3850 flash via USB or TFTP.

Step 2: Check the mode (Install or Bundle) of the switch by entering following command:

Switch#show version | begin Switch Ports

The out put will be as below:

Note: If the Switch is not in INSTALL Mode it’s highly recommended to switch INSTALL mode from BUNDLE mode. Click here to find out how or enter the below commands:

# software expand running to flash:
(config)#boot system flash:packages.conf

Step 3: Enter the following command to start installing the new IOS using the new code on the flash

Switch#:software install file flash:<New_IOS_File_Name>.bin

Example:
Switch#:software install file flash:cat3k_caa-universalk9.SPA.03.03.00.SE.150-1.EZ.bin

Then following will be shown:

Step 4: enter yes and hit enter.

NOTE: Ran into some issue booting 3850. This might be helpful if your Catalyst 3850 Fails to Boot.

3850-access#show boot
—————————
Switch 1
—————————
Current Boot Variables:
BOOT variable = flash:packages.conf;

Boot Variables on next reload:
BOOT variable = flash:packages.conf
Allow Dev Key = yes
Manual Boot = yes
Enable Break = no

Notice it’s set to Manual Boot = yes

you need to change that to NO

#conf t

(config)#no boot manual

#reload

If this also doesn’t fix the boot issue refer the following blog post here.