You will need to sell from one product into cash, then use that cash to invest into the new product.
For example if you hold fund A in your ISA and you would like to switch that into share XYZ plc you would need to do the following;
Sell the fund in your ISA and request that the cash remains within your ISA, i.e. do not request it to be paid out to your bank account.
You will then need to wait for FundsNetwork to receive confirmation from the fund provider as to what the sale proceeds will be. This could take up to a day or more from you submitting the sale instruction, it will depend on the time that you placed the sale and the time that the fund being sold is priced. We would suggest that you check your account every few hours if you are very keen to invest the sale proceeds.
After FundsNetwork have received confirmation of the sale proceeds they will update the available cash in your ISA. You will then be able to invest that cash and Buy your chosen share.
Similar rules would apply if you were doing the opposite, i.e. selling a share and investing that into a fund.