I have been here
In this two sentences, I'm confused "here" is whether adjective or adverb. As I know linking verbs follow adjective not adverb, it (here) would be adjective,isn't it?
In these contexts it is normally analysed as an adverb, meaning "at this place".
Adverbs are a bit of a mixed bag of words. There are various ways of analysing the sentence "I'm here". You can say that "here" is the subject complement, or you can say that the verb "'m" is not a linking verb, but a verb meaning "to exist". Or you can treat this kind of expression as an idiom.
Note you can also have other location and time adverbs/adverbial phrases in a similar position
The show was here.
The show was on the field
The show was yesterday
The show was at 5pm