It shouldn't be hard to picture. For example, lets taking adding new information to the cell. How does this work? Well, normally during the process of cell division the cell will replicate it's D.N.A in a process called replication, the cell will sometimes accidentally add new information by replicating extra D.N.A or bases.

[link to www.nature.com]

The above process is the main mechanism behind evolution. There are other ways D.N.A is changed and mutates to become new, different D.N.A, but for the most part replication errors are the main factors behind mutation. MOST of the time, the mutations are either harmful or have no effect, and sometimes are good (such as lactose tolerance.)