You don't mention the version of Spectrum that you are on and wanting to update. However, there are two types of updates that are available for Spectrum out of the main release cycle (as in the major/minor releases). These are:
- BMP updates (Bi-Monthly Patch)
- Certification updates
They are available from here:
ftp://ftp.ca.com/CAproducts/CA-SPECTRUM/Updates/GA/
By default, in a standard Spectrum installation, OOTB there are NO custom MIB files present. Therefore, the only way that you are going to get custom MIBs is if you import them. Generally this activity is only performed if you are doing Spectrum upgrades and comparing as to what was discovered under an old Spectrum version compared to the latest Spectrum release (where there is probably normal or extended support introduced for a device).
I should also state that adjusting MIBs is NOT like updating software (as in updating Apps on your mobile). There is no real desire to go to the latest/greatest MIBs for a device unless you absolutely need an extra OID that isn't present under the standard setup. Also, network management tools aren't really designed to behave like this since you will generally have to take them down in order to introduce a MIB update (which is not what you want for a monitoring system).
Importing MIBs isn't a trivial task. I would have a look here first of all:
https://docops.ca.com/ca-spectrum/10-3-1/en/customizing/model-type-editor/import-mibs