Do:
p4 resolve -as
from the command line. In the vast majority of cases that's all you need to do; it tells resolve to accept the change relative to the base (which means accepting the source in the case where the target hasn't changed).
(updating to include additional info from comment)
If after you do that you get an error like:
can't move (open for delete); must accept other resolve(s) or ignore
it means that the file was moved in the source (which means that normally a resolve -as would move the workspace file to match it), but the file in your workspace can't be moved because it's already open for delete (you can't move a deleted file). This is a pretty rare situation that happens if you move a file, delete it, and then try to resolve those two operations independently (but without submitting in between). In that case you probably want to specifically "ignore" the move resolve (like the error message suggests) by doing:
p4 resolve -ay
If you've somehow gotten your working file into a bad state (maybe you were running random commands before setting up the integrate and the workspace wasn't in a clean state) and either can't figure out how you got here or have no interest in doing forensics when you just want to do a basic integrate and forget about whatever you were in the process of doing before, you can always start over like this:
p4 revert FILE
p4 integrate -b BRANCH FILE
p4 resolve -as
