Hi, With the context you have offered, my guess is that it simply means 'Return to a higher position'. eg Did person B recently jump down from the top of the wall. If so, the phrase means 'Get back up on top of the wall'.
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No, get back up does not mean lean against the wall. It means when you have a downfall, you got to literally get back up. Not in an "Imma get back at you" kind of way, but keep trying no matter what gets in your way.