diff --git a/pyacl/acl.py b/pyacl/acl.py
index ca32bb6..86af0d7 100644
--- a/pyacl/acl.py
+++ b/pyacl/acl.py
@@ -243,6 +243,46 @@ def apply_acl_to_path(acl, path):
   acl.applyto(path)
 
 
+def merge_acls(acl1, acl2):
+  """
+  Example usage: merge_acls(posix1e.ACL, posix1e.ACL)
+  Return: posix1e.ACL
+  """
+  acl3 = ACL(acl=acl1)
+  for entry in acl2:
+    tag_type = entry.tag_type
+
+    # keep existing entries which may only exist once
+    if tag_type not in [ACL_USER_OBJ, ACL_GROUP_OBJ, ACL_OTHER]:
+
+      # replace existing user/group entries with new ones if the uid/gid matches
+      if tag_type in [ACL_USER, ACL_GROUP, ACL_MASK]:
+
+        for existing_entry in acl3:
+          existing_tag_type = existing_entry.tag_type
+
+          if tag_type in [ACL_USER, ACL_GROUP, ACL_MASK]:
+            if tag_type == existing_tag_type:
+              if tag_type == ACL_MASK or entry.qualifier == existing_entry.qualifier:
+                acl3.delete_entry(existing_entry)
+
+      acl3.append(entry)
+
+  acl3.calc_mask()
+
+  return acl3
+
+
+def update_acl_on_path(new_acl, path):
+  """
+  Example usage: update_acl_on_path(posix1e.ACL, '/etc/foo.txt')
+  Return: None
+  """
+  acl = merge_acls(read_acl_from_path(path), new_acl)
+
+  return apply_acl_to_path(acl, path)
+
+
 def read_acl_from_path(path):
   """
   Example usage: read_acl_from_path('/etc/foo.txt')
diff --git a/tests/matrix-apply-update.yaml b/tests/matrix-apply-update.yaml
new file mode 100644
index 0000000..2f6e2a5
--- /dev/null
+++ b/tests/matrix-apply-update.yaml
@@ -0,0 +1,24 @@
+---
+user:user:rw:
+  args:
+    target_name: user
+    target_type: user
+    read: false
+    write: true
+  expect:
+    user:
+      user:
+        read: false
+        write: true
+        execute: false
+    group: &null_allfalse
+      null:
+        read: true
+        write: false
+        execute: false
+    mask: &null_ro
+      null:
+        read: true
+        write: true
+        execute: false
+    other: *null_allfalse
diff --git a/tests/test_pyacl.py b/tests/test_pyacl.py
index 73134ce..fd28b2d 100644
--- a/tests/test_pyacl.py
+++ b/tests/test_pyacl.py
@@ -23,6 +23,19 @@ def load_yaml(file):
   return list(data.items())
 
 
+def load_yamls(file1, file2):
+  data1 = load_yaml(file1)
+  data2 = load_yaml(file2)
+
+  out = []
+
+  for lentry in data1:
+    for i in range(len(lentry)):
+      ix = i - 1
+      out.append( ({'first': data1[ix], 'second': data2[ix]}) )  # noqa UP034, pytest mangles this in the parameters
+
+  return out
+
 @mark.parametrize('aclin, aclout', load_yaml('matrix.yaml'))
 def test_parse_acl_through_string(sample_file_with_acl, aclin, aclout):
   have = acl.parse_acl_from_path_via_string(sample_file_with_acl)
@@ -35,10 +48,36 @@ def test_parse_acl_native(sample_file_with_acl, aclin, aclout):
   assert aclout == have
 
 
+@mark.parametrize('mode', ['fresh', 'update'])
 @mark.parametrize('scenario, data', load_yaml('matrix-apply.yaml'))
-def test_build_and_apply_acl(sample_file, scenario, data):
+def test_build_and_apply_acl(sample_file, mode, scenario, data):
   built_acl = acl.build_acl(**data['args'])
   assert len(list(built_acl)) == 5  # noqa PLR2004, this is the expected size of the built ACL
   assert acl.apply_acl_to_path(built_acl, sample_file) is None
+  if mode == 'update':
+    assert acl.update_acl_on_path(built_acl, sample_file) is None
   read_acl = acl.parse_acl_from_path(sample_file)
   assert read_acl == data['expect']
+
+
+@mark.parametrize('data', load_yamls('matrix-apply.yaml', 'matrix-apply-update.yaml') )
+def test_build_and_update_acl(sample_file, data):
+  # we're updating the default instead of overwriting with apply
+  # one better way would be to have both "before" and "after" in matrix-apply-update.yaml instead of re-using the one for apply
+  # another would be to somehow adjust the apply map
+  data['first'][1]['expect']['group'][None]['read'] = True
+  data['first'][1]['expect']['other'][None]['read'] = True
+
+  built_acl1 = acl.build_acl(**data['first'][1]['args'])
+  assert len(list(built_acl1)) == 5  # noqa PLR2004, this is the expected size of the built ACL
+
+  built_acl2 = acl.build_acl(**data['second'][1]['args'])
+  assert len(list(built_acl2)) == 5  # noqa PLR2004
+
+  assert acl.update_acl_on_path(built_acl1, sample_file) is None
+  read_acl1 = acl.parse_acl_from_path(sample_file)
+  assert read_acl1 == data['first'][1]['expect']
+
+  assert acl.update_acl_on_path(built_acl2, sample_file) is None
+  read_acl2 = acl.parse_acl_from_path(sample_file)
+  assert read_acl2 == data['second'][1]['expect']