A battery consists of 10 cells, each of emf 1V if 2 cells are wrongly connected the emf of the battery becomes

When cells are coupled in a series circuit configuration, their electromotive forces (emfs) add algebraically based on orientation. If a cell is reversed or wrongly connected, its potential acts in opposition to the correct configuration.
Out of $10$ cells, each with an emf of $1\text{V}$:

* Number of correctly connected cells = $10-2=8$
* Number of wrongly connected cells = $2$

Each wrongly connected cell cancels out its own voltage as well as the voltage of one correctly connected cell. The net effective emf ($E_{\text{net}}$) is:

$$E_{\text{net}}=(8\times 1\text{V})-(2\times 1\text{V})=8\text{V}-2\text{V}=6\text{V}$$