Funds are generally split into units, and when you invest in a fund, you are buying units in that fund. If the fund performs well, you can expect the value of your units to increase and that is how you will profit from an investment fund. Ultimately, to make a profit, the unit price at point of sale should be higher than the unit price at point of initial purchase.
In terms of when to sell a fund; there are lots of variable to consider. These include your need for cash, personal circumstance, investment strategy and overall faith in the fund. For example, consistent poor performance in the fund and continual changing of the fund manager may be strong enough reasons for someone to sell their fund.