In some cases, the categories in a demographic question are more detailed than the ones you wish to use for weighting. An example is a question about the respondent's Age which stores the age in categories of five years (e.g. 19 to 14, 25 to 29, etc), but where the data needs to be weighted according to aggregated 10-year groups (eg 19 to 29, etc).
Method - Automated
You can merge together the categories on a table and use an automation to save down a copy of the question with the merged categories, see How to Make Merged Categories Appear in Targets when Weighting.
Method - Ad hoc/manual
Follow the normal process for creating a weight and drag and drop the categories on top of one another to merge in the table of targets.