Is there any language which is known to have acquired verb inflectional morphology in the process of evolution?Languages like English have given up inflections on verb,but I havent heard about any which has acquired the PNG inflection on verb.Is there any?

Is there any Historical evidence for that,other than speculations

In the Uralic family, some members of the Samoyed branch, e.g. Nenets, are well-known for their highly complex verbal morphology. The current view is that these forms can't stem from Proto-Uralic and must be a later innovation. Of course, there is no attested evidence since there is a lack of early written tradition in much of the Uralic languages; the evidence comes from comparative analysis instead. For example, take a look at this grammatical sketch of Tundra Nenets by Tapani Salminen.

It seems that there are lot more evidence for languages giving up inflectional morphology than developing new.  Does that imply language change involves simplification?

Well, people only really focus on the obvious simplifications. There's a lot of rebuilding and restructuring that happens. Just look at a random selection of PIE languages now, knowing that they developed into such divergent structures is direct proof that verbal morphology takes on all sorts of weird and wonderful twists and turns.

The process is called grammaticalisation and it's a new and rapidly growing field in linguistics. Basically, verbal endings usually start out as content words and then grammaticalise into structural words that live on the end of stems as grammatical endings.

Taking a quick route down the Italic branch of PIE just for an example, you can see in Latin the forms for the word 'have' eroded away and became the future tense in modern Italian today:

amare + habeo = amerò
amare + habet = amerà

The original Latin for "I will love" was 'amabo' which going further back to PIE comes from 'am + bhwo' which is the root of the word plus the verb 'to be'. There is definitely a visible trend for some languages to move into more isolating/analytic morphology, which rely less on specific endings but have all sorts of other ways to play with tense ideas. The English form 'will' used to mean wish/want and then it grammaticalised and suddenly saying that you wish for something took on the meaning of intention and thus developed into a future marker today.

So, yes in some cases it involves simplification and in other cases it just involves altering the structure. In other cases it involves a rise in complexity. Some languages are definitely more expressive in their ability to easily use moods like the subjunctive to make fine-grained distinctions in interpretation of language which would sound clumsy, too-wordy and generally unnatural if the exact same semantics were to be attempted in a language without that easy-access to such possibilities.

Some languages seem to like going in one direction, but it's an incomplete pool of data to look at PIE languages and the trends visible there, and from that, deduce that language change involves simplification, when just out of the corner of the analysis window there is a language like Archi which can theoretically change a verb into over 1.5 million different forms. That kind of complexity isn't typical. Its complexity lies is language change.

To sum up the point, language change is exactly that - language change.

It seems that there are lot more evidence for languages giving up inflectional morphology than developing new.  Does that imply language change involves simplification?

As a well-known historical linguist used to say in his seminars, "Languages always trend towards simplicity, except for when they get more complex."