First, download the appropriate firmware from the CISCO website. You will need to be a registered user to do this. In most cases you want the "Advanced IP Services" image.

If you are using minicom, make sure you have the lrzsz package installed, which implements the ymodem implementation we use below.

>
> enable 
# terminal speed 115200
# copy ymodem: flash:filename
# configure terminal
(config)# boot system flash:filename
(config)# exit
# copy running-config startup-config
# reload

If the image you downloaded is a tarball, you will need to untar it once it's on the switch. Note that the filename that should be used for the boot command is that of the .bin image that will be extracted:

# archive tar /xtract flash:tarball

If the flash filesystem is full, you may need to do something like this to clear space:

#dir flash:
#delete flash:filename