can plzzz explain the mechanism of isomerism in Cis -butene?

First, Rajashekar there may be a incompletion in your question because there is no such compound Cis-Butene exist in organic Chemistry and the reason behind its non Existence is the double bond attach with C1-C2 and C1 havinf two H , C2 having one H so for containing cis or trans isomer there must be two different group attach on particular carbon rather than having same group like C1 having in this situation.

Now, If you are talking about Just Butene then isomers of butene is :

1. Normally we represent Butene as in == But-1-ene

2. By changing the position of Double Bond We can get == But-2-ene

3. By changing the way the carbon - carbon attached with each other we can get == 2-methylpropene

4. For But-2-ene We can get two Geometrical isomer :

cis-2-butene ((Z)-2-butene)      [ By having two bulkies group in the same side on both Carbon atom connected with double bond]

trans-2-butene ((E)-2-butene).  [ By having two bulkies group in the same side on both Carbon atom connected with double bond]