Excel problem: I need to let only values between a certain range be in a column and if they are less

It's actually related to well log. So upto a certain depth (1000 for example) our hydrostatic head (column h) remains as a constant value of 8.3. After that it depends on other factors so it's basically equal to some other cell value (for example g). Now.. I want my graph to be smooth. Column h has few points which are below 8.3 or 20 (max value). And if a value is 5 for example then I want the value to become 8.3 and if it's 25 I want it as 20 in my graph.