XML projection is one of the main adopted optimization techniques for reducing memory consumption in XQuery in-memory engines. The main idea behind this technique is quite simple: given a query Q over an XML document D, instead of evaluating Q on D, the query Q is evaluated on a smaller document D′ obtained from D by pruning out, at loading-time, parts of D that are unrelevant for Q. The actual queried document D′ is a projection of the original one, and is often much smaller than D due to the fact that queries tend to be quite selective in general. While projection techniques have been extensively investigated for XML query- ing, we are not aware of applications to XML updating. This Thesis investigates application of a projection based optimization mechanism for XQuery Update Facility expressions in the presence of a schema. The current work includes study of the method and a formal development of Merge algorithm as well as experiments testifying its effectiveness.