I assume you mean you want to change the Title of the Source. Any changes you make to one Source will not affect any others. However if you have cited that source more than once all the citations will still correctly link to the right source. (For example you might have cited a marriage certificate as a source for the marriage event itself but also for the occupations of the bride and groom, and for their fathers’ names and occupations.)
If you end up with duplicate source names FH will be able to cope because it distinguishes them by their record ID. But it might be a sign that you have actually recorded the same source twice, originally under different titles, so you should check that and if necessary the sources can be merged.
I need to standardise my 'sources' as, over time, they have lost some consistency. I can't seem to find the answer in the FHUG knowledge base.
I THINK I can change them this way (but I do have thousands of records, so want to be absolutely sure before I commit):
Go to records window, click on 'sources', click on the name of the source which opens a 2nd window. I can then make changes in this window. However, does this change ALL the relevant sources? If it does, what happens if there is then a duplication of source names?