ubuntu sync error in china

Err:20 http://mirrors.ustc.edu.cn/ubuntu focal-updates/main amd64 c-n-f Metadata
  File has unexpected size (5152 != 5172). Mirror sync in progress? [IP: 202.38.95.110 80]
  Hashes of expected file:
   - Filesize:5172 [weak]
   - SHA256:e01d3d12b5745f05a388866bdd556a25f1f387b18cb604ac61f197e7992261bb
   - SHA1:cd38ba779930f2c2c455d9a25134abee87ae575e [weak]
   - MD5Sum:b75556b7010e9bd0cecfda9afd853f3c [weak]
  Release file created at: Wed, 03 Jun 2020 14:01:16 +0000
Hit:26 http://archive.ubuntu.com/ubuntu focal InRelease
Fetched 265 kB in 3s (98.2 kB/s)
Reading package lists... Done
E: Failed to fetch http://mirrors.ustc.edu.cn/ubuntu/dists/focal-updates/main/cnf/Commands-amd64.xz  File has unexpected size (5152 != 5172). Mirror sync in progress? [IP: 202.38.95.110 80]
   Hashes of expected file:
    - Filesize:5172 [weak]
    - SHA256:e01d3d12b5745f05a388866bdd556a25f1f387b18cb604ac61f197e7992261bb
    - SHA1:cd38ba779930f2c2c455d9a25134abee87ae575e [weak]
    - MD5Sum:b75556b7010e9bd0cecfda9afd853f3c [weak]
   Release file created at: Wed, 03 Jun 2020 14:01:16 +0000
E: Some index files failed to download. They have been ignored, or old ones used instead.
Reading package lists... Done
Building dependency tree