Hi,
First and foremost you'll need to obtain a copy of a recent ASA image file. You should also consider getting a new ASDM image at the same time. This can be acquired from cisco.com's download section either if your device was a direct purchase from cisco.com, are CCIE certified, have a service contract with them or if you're an authorized reseller of their products. Once you have the image, copy it over to your device using one of many ways (SCP if enabled, tftp, ftp). Personally I like TFTP. Once the
image has been copied over to disk0, you can issue the following command at the enable prompt, under config mode:
boot system config disk0:/image.bin
And of course replace image.bin by the name of the image you just uploaded. You also should update the ASDM's image at the same time. This can be accomplished with:
asdm image disk0:/image.bin
where image.bin is the name of the image you just uploaded.