If you're comfortable with basic work, there should be tutorials all over the place that cover it.
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More than likely, you have changed string gauges to a set with less tension and you're not getting the same load on the neck. Your truss rod probably needs to be adjusted. If you're not comfortable with basic tech work, take it to a shop to have it adjusted... most places will do this when they change the strings for you!
If you're comfortable with basic work, there should be tutorials all over the place that cover it.
If you're comfortable with basic work, there should be tutorials all over the place that cover it.