Update changelog.md

for release 2.6.3.

changelog.md

@@ -1,5 +1,17 @@
 # Changelog (from 22/07/2021)
 
+## 22/12/2021 - version 2.6.3. 
+### Frontend ([PR #272-281](https://github.com/xchem/fragalysis-frontend/pull/272)). 
+:heavy_plus_sign: GUI improvements
+:heavy_plus_sign: Data download improvements/fixes
+:heavy_plus_sign: Tag editionr improvements
+:heavy_plus_sign: Snapshopt bug fixes
+### Backend ([PR #292](https://github.com/xchem/fragalysis-backend/pull/292)). 
+:heavy_plus_sign: Data download improvements/fixes
+
+### Resolved tickets. 
+[#752](https://github.com/m2ms/fragalysis-frontend/issues/752), [#689](https://github.com/m2ms/fragalysis-frontend/issues/689), [#749](https://github.com/m2ms/fragalysis-frontend/issues/749), [#550](https://github.com/m2ms/fragalysis-frontend/issues/550), [#685](https://github.com/m2ms/fragalysis-frontend/issues/685), [#696](https://github.com/m2ms/fragalysis-frontend/issues/696), [#735](https://github.com/m2ms/fragalysis-frontend/issues/735), [#705](https://github.com/m2ms/fragalysis-frontend/issues/705), [#708](https://github.com/m2ms/fragalysis-frontend/issues/708), [#687](https://github.com/m2ms/fragalysis-frontend/issues/687), [#690](https://github.com/m2ms/fragalysis-frontend/issues/690), [#730](https://github.com/m2ms/fragalysis-frontend/issues/730), [#701](https://github.com/m2ms/fragalysis-frontend/issues/701), [#740](https://github.com/m2ms/fragalysis-frontend/issues/740), [#677](https://github.com/m2ms/fragalysis-frontend/issues/677), [#686](https://github.com/m2ms/fragalysis-frontend/issues/686), [#713](https://github.com/m2ms/fragalysis-frontend/issues/713), [#706](https://github.com/m2ms/fragalysis-frontend/issues/706), [#704](https://github.com/m2ms/fragalysis-frontend/issues/704), [#651](https://github.com/m2ms/fragalysis-frontend/issues/651), [#680](https://github.com/m2ms/fragalysis-frontend/issues/680), [#729](https://github.com/m2ms/fragalysis-frontend/issues/729), [#674](https://github.com/m2ms/fragalysis-frontend/issues/674)
+
 ## 8/12/2021 - version 2.6.2. 
 ### Frontend ([PR #269-271](https://github.com/xchem/fragalysis-frontend/pull/269)). 
 :heavy_plus_sign: Fixed select the first molecule