There's a few options here depending on what you need from the field. You can uppercase the field in the browser, at request time, or as data is ingested into elasticsearch.
if you want it done in the browser you can do this with a field formatter, accessible from the index pattern pages:
If you want it done at request time in elasticsearch, you can create a scripted field, likewise done in the index patterns page:
Independent of kibana, you can upper case the field as data is ingested into elasticsearch using the uppercase processor. All values will be stored in upper case and flow through to kibana as such